Late Pleis to cene and Ho lo cene en vi ron men tal evo lu tion of the Wkra River Val ley near Bielawy Go³uskie (cen tral Po land) re corded in palaeo-ox bow lake de pos its
Monika NISKA1, *, Jerzy JONCZAK2 and Joanna GADZISZEWSKA1
1 Pom er a nian Uni ver sity in S³upsk, In sti tute of Ge og ra phy and Re gional Stud ies, Partyzantów 27, 76-200 S³upsk, Po land
2 War saw Uni ver sity of Life Sci ences, De part ment of Soil En vi ron ment Sci ences, Nowoursynowska 159, 02-776 Warszawa, Po land
Niska, M., Jonczak, J., Gadziszewska, J., 2017. Late Pleis to cene and Ho lo cene en vi ron men tal evo lu tion of the Wkra River Val ley near Bielawy Go³uskie (cen tral Po land) re corded in palaeo-ox bow lake de pos its. Geo log i cal Quar terly, 61 (2):
305–318, doi: 10.7306/gq.1320
This study aimed to re con struct en vi ron men tal changes in the Wkra River Val ley near Bielawy Go³uskie (cen tral Po land) based on geo chem i cal, palynological and subfossil Cladocera anal y sis of de pos its fill ing a palaeo-ox bow lake. Two sed i - ment bore holes col lected from the palaeochannel bend (BG-1) and neck (BG-2) in clude full se quences of or ganic de pos its ly ing on clastic de pos its. The lower part of the BG-1 bore hole con tains gyttja that ac cu mu lated dur ing the Youn ger Dryas and the Preboreal, which is cov ered by reed peat of Preboreal and Bo real age. On top of this is al der peat that ac cu mu lated dur - ing the At lan tic. The ages of the gyttja and peat were con firmed by both ra dio car bon dat ing and palynological anal y sis. The peats are char ac ter ized by a sig nif i cant con tent of or ganic mat ter, which is rel a tively sta ble over time. This in di cates a low in - ten sity of me chan i cal de nu da tion in the catch ment, which is sup ported by low con cen tra tions of alu mi num and po tas sium.
Cal cium val ues greatly ex ceed these el e ments, sug gest ing that the lake was mainly fed by ground wa ter. Vari a tion in trophic con di tions over time is in di cated by a ver ti cal vari abil ity in the con tent of ni tro gen, as well as by the Cladocera pop u la tion. The strong de com po si tion of peat in the top layer of the mire, and in creased con cen tra tions of phos pho rus and heavy met als, tes - tify to in creas ing hu man ac tiv ity dur ing the most re cent time pe ri ods.
Key words: ox bow lake, Wkra River, Po land, Multi-proxy anal y sis, Early Ho lo cene.
INTRODUCTION
The de vel op ment of lakes, con sti tut ing im por tant post - glacial land scape el e ments (Kalinowska, 1961), is strongly as - so ci ated with a basal fill of strat i fied de pos its that are char ac ter - ized by vari able physico-chem i cal prop er ties over time. The ac - cu mu la tion re sults in fill ing and fi nally terrestrialization of the res er voir. The in ten sity of this pro cess and the na ture of the sed i ments are af fected by many en vi ron men tal fac tors, in clud - ing e.g. cli ma tic con di tions, in ten sity of me chan i cal and chem i - cal de nu da tion in the catch ment, its cover, spe cies com po si tion and fre quency of lake or gan isms (Marszelewski et al., 2011). In this way, lake de pos its con sti tute ar chives of past en vi ron ments of the res er voir and its catch ment that can be re con structed by palaeo eco logi cal stud ies.
Geo chem i cal anal y sis of sed i ments is im por tant for palaeoenvironmental re con struc tions, es pe cially the in ves ti ga - tion of pH and the con tent of ba sic com po nents in clud ing sil ica, car bon ates, or ganic mat ter and some el e ments, par tic u larly C,
N, P, K, Ca, Mg, Fe, Al, Mn, Cu, Zn, Ni and Pb (Wojciechowski, 2000; Borówka, 2007). Fur ther more, valu able anal y ses in clude speciation of el e ments, the con tent of their iso topes, or qual i ta - tive and quan ti ta tive anal y sis of spe cific or ganic and in or ganic com pounds (Herczeg, 1988; Martinova, 1993; Meyers and Lallier-Vergès, 1999; End ers and Pagani, 2008). In many stud - ies, the in ter ac tions be tween physico-chem i cal pa ram e ters of the sed i ments and en vi ron men tal con di tions in the lakes and their catch ments have been iden ti fied. For ex am ple, the con - tents of sil ica, po tas sium and alu mi num are com monly con sid - ered as in di ca tors of me chan i cal de nu da tion in ten sity, whereas the con tent of cal cium re flects the in ten sity of chem i cal de nu da - tion. The abun dance of ni tro gen and phos pho rus pro vides in for - ma tion about trophic con di tions; con cen tra tions of heavy met - als are in di ces of hu man ac tiv ity (Ham il ton-Tay lor, 1979; Abra - ham, 1998; Last and Smol, 2001; Rose et al., 2004; Jin et al., 2006; Borówka, 2007; Schmidt et al., 2008; Paw³owski et al., 2015a, b). How ever, pri mary fea tures of sed i ments can un - dergo trans for ma tion in post-sed i men ta tion pro cesses (Boyle, 2001; Hag gard et al., 2005; Wil son et al., 2008), which may ham per cor rect in ter pre ta tion.
In te grat ing the re sults of geo chem i cal anal y sis and the com po si tion and fre quency of var i ous mi cro or gan isms pre - served in sed i ments pro vides a much more com plete pic ture of past en vi ron ments (Galbarczyk-G¹siorowska et al., 2009;
Paw³owski et al., 2015a, b). Pol len anal y sis, es pe cially when
* Corresponding author, e-mail: monikaniska@wp.pl
Received: January 1, 2016; accepted: April 26, 2016; first published online: January 31, 2017
com bined with ra dio car bon dates, plays a key role in re con - struct ing the past veg e ta tion of the palaeo-lakes, al low ing the es tab lish ment of a chro nol ogy of the de pos its (Lata³owa, 1982; Lata³owa and Tobolski, 1989). Past eco log i cal con di - tions of the ba sin can fur ther more be re con structed from malacological anal y ses (Wojciechowski, 1987, 2000) and Cladocera anal y sis (Miros³aw-Grabowska and Niska, 2005;
Miros³aw-Grabowska et al., 2009; Niska, 2013). The value of these groups of or gan isms is re lated to the fact that many spe - cies are adapted to nar row en vi ron men tal ranges. Cladocera, which are the ma jor con stit u ents of lake zoo plank ton, in habit mostly stag nant lakes (Kajak, 2001), par tic u larly un der con di - tions of a sta ble lake level (Lampert and Sommer, 2001), in - clud ing ox bow lakes (Dembo wska and Napiórkowski, 2012).
How ever, en vi ron men tal re con struc tions of palaeo-ox bow lake de pos its may be dif fi cult due to the sig nif i cant dy nam ics of river val ley geosystems and flu vial pro cesses, which can dis rupt sed i men ta tion, lead ing to the for ma tion of strati graphic hi a tuses (Paw³owski et al., 2015a).
Our study aims to bring new in for ma tion on past en vi ron - ments in the Wkra River Val ley near Bielawy Go³uskie (cen tral Po land) based on the anal y sis of geo chem is try, pol len and subfossil Cladocera of palaeo-ox bow lake de pos its. The re con -
struc tion in cludes the chro nol ogy of ox bow lake for ma tion and its cut-off from the river, the tem po ral vari abil ity of trophic con di - tions in the lake, the in ten sity of me chan i cal and chem i cal de nu - da tion in the catch ment, the suc ces sion of veg e ta tion and Cladocera, and the iden ti fi ca tion of hu man ac tiv i ties against the back ground of past cli mate changes.
MATERIALS AND METHODS
STUDY AREA
The stud ies were car ried out in a palaeo-ox bow lake on the east side of the Wkra River Val ley near the vil lage of Bielawy Go³uskie (52°93.7270’ N, 19°96.6163’ E; Fig. 1). The area is lo - cated at the mar gins of the Raci¹ska Plain (Kondracki, 2000), which is a glaciofluvial pla teau that de vel oped dur ing the max i - mum ex pan sion of the Vistula Gla ci ation, ly ing at the el e va tions of 113–121 m a.s.l. The glaciofluvial land scape con tains out - crops of older gla cial tills that are lo cally cov ered by coarse - -grained re sid ual ma te ri als with stones and boul ders, Late Pleis to cene and Ho lo cene dunes, and ket tle holes of vari able
Fig. 1. Lo ca tion of study site and bore holes within the palaeo-ox bow lake
sizes that are filled with peat and gyttja (Kotar biñski, 2003). This land scape is cut by the ~1 km wide Wkra River val ley. The bot - tom of the val ley is filled with al lu vial de pos its with thick nesses of up to ~4 m. Many palaeo channels in the val ley con sti tute ev i - dence of the past ac tiv ity of a me an der ing river. The age and stage of the palaeo channels are vari able. The old est are prob a - bly the large palaeo -ox bow lakes along the val ley mar gins that at pres ent are com pletely or al most com pletely filled with or - ganic de pos its. Drain age works within the val ley bot tom and straight en ing of the river chan nel dur ing the mid-20th cen tury de creased the ground wa ter level and led to the dis ap pear ance of some wa ter bod ies.
SAMPLING
Two sed i ment bore holes with a length of 2 m were taken from the palaeo-ox bow lake bend (BG-1) and neck (BG-2) (Fig.
1) us ing a 75 mm Instorf corer, cov er ing the com plete lake fill and the up per lay ers of the un der ly ing clastic de pos its. Sam - ples were taken for geo chem i cal anal y ses (5 cm sam ple in ter - val), palynological stud ies (sam ple in ter val 8 cm), Cladocera anal y sis (8 cm sam ple in ter val) and ra dio car bon anal y sis (se - lected lev els).
RADIOCARBON DATING
Ra dio car bon dates were ob tained from plant re mains col - lected at three depths in bore hole BG-1. Dat ing was per formed in the Lab o ra tory of Ab so lute Dat ing in Cianowice. Con ven - tional dates were cal i brated us ing OxCal 4.2.3 soft ware (Bronk Ramsey, 2013) and the IntCal13 cal i bra tion curve (Reimer et al., 2013). Here, dates are given in cal i brated years BP.
POLLEN ANALYSIS
Sam ples of 1 cm3 vol ume were boiled in 10% KOH, treated with 10% HCl or soaked for sev eral days in HF (de pend ing on the sed i ment com po si tion) and then acetolysed (Faegri and
Iversen, 1989; Dybova-Jachowicz and Sadowska, 2003). As a rule, in each sam ple 700 pol len grains of tree and shrub pol len (AP) and all ac com pa ny ing pol len of herbs (NAP) and spores of Pteri do phytes and mosses were de ter mined and counted. Cal - cu la tions were based on AP + NAP = 100%; pol len and spores of aquatic and wet land spe cies are ex cluded from the cal cu la - tion sum. Also, microremains of al gae, Cyanobacteria, fungi and some fauna (non-pol len palyno morphs – NPPs) were counted (Van Geel, 2001; Jankovská and Komárek, 2001). The re sults of pol len anal y sis (Figs. 2 and 3) are dis played us ing the PolPal soft ware (Nalepka and Walanus, 2003).
GEOCHEMICAL ANALYSIS
Sam ples were dried at 65°C, pul ver ised with an an a lyt i cal mill with a tung sten blade (or ganic sam ples) or ag ate mor tar (min eral sam ples) and stored in closed poly eth yl ene bags. The anal y sis in cluded:
– de ter mi na tion of pH potentiometrically in sus pen sion with wa ter (1:10);
– de ter mi na tion of car bon ate con tent with the vol u met ric Scheibler’s method;
– de ter mi na tion of or ganic mat ter con tent (OM) as loss- on -ig ni tion at 550°C;
– de ter mi na tion of to tal or ganic car bon (TO) con tent with the method of Alten et al. (1935);
– de ter mi na tion of to tal ni tro gen (N) con tent with Kjeldahl’s method (Page, 1982);
– de ter mi na tion of the con tent of P, Na, K, Ca, Mg, Fe, Al, Mn, Cu and Zn af ter sam ple di ges tion in a mix ture of 65% HNO3, 60% HClO4 and con cen trated H2SO4 in the pro por tion 20:5:1 by vol ume. The con cen tra tion of P in so lu tions was ana lysed colorimetrically by the mo lyb de - num blue method, and the re main ing el e ments by mi cro - wave plasma atomic emis sion spec trom e try (Agilent 4100 MP-AES).
Based on the chem is try of bore hole BG-1, five geo chem i cal zones were iden ti fied us ing clus ter anal y sis with PAST 3.10 soft ware (Fig. 4; Ham mer et al., 2001).
Fig. 2. Per cent age pol len di a gram of bore hole BG-1
CLADOCERA ANALYSIS
Sam ples with a vol ume of 1 cm3 were pre pared us ing a slightly mod i fied stan dard pro ce dure (Frey, 1986; Korhola and Rautio, 2001). Af ter re moval of car bon ates us ing 10% HCl, each sam ple was boiled in 10% KOH for 20 min utes, washed with dis tilled wa ter and sieved through a 40 mm mesh sieve.
The fine ma te rial was trans ferred into a polycarbon test tube.
Prior to count ing, the re mains were col oured with safranine T.
The anal y sis was per formed with a Nikon model ECLIPSE Ci-L mi cro scope with mag ni fi ca tions of x10, x40 and x60. A min i - mum of 200 re mains of Cladocera (3–8 slides) were ex am ined in each sam ple. First, all re mains from each slide were counted (headshields, shells, postabdomens, postabdominal claws and an ten nules) and then con verted to one Cladocera spec i men and all ephippia to gether.
Iden ti fi ca tion and eco log i cal in ter pre ta tion of the Cladocera re mains were based on Frey (1958, 1962), Goulden (1964), Szeroczyñska (1985, 1998), Hofmann (1986, 2000), Korhola (1990), Duigan (1992), Flössner (2000) and Szeroczyñska and Sarmaja-Korjonen (2007). The re sults of Cladocera anal y ses are shown in Fig ures 5 and 6, and in clude the per cent age di a - gram, the to tal num ber of Cladocera in di vid u als, and num ber of spe cies in clud ing their biodiversity. The nu mer i cal anal y sis was per formed with the PolPal com puter pro gram (Nalepka and Walanus, 2003).
The Cladocera spe cies were clas si fied into four hab i tat pref er ence groups: bot tom-dweller spe cies, spe cies as so ci ated with/re stricted to veg e ta tion and plank tonic (off shore) and lit to - ral (meiobenthic) groups (Flössner, 1964; White side, 1970;
White side and Swindoll, 1988; Korhola, 1990).
RESULTS
LITHOLOGY AND AGE OF DEPOSITS
The ra dio car bon dates show that the sed i ments were de pos - ited be tween the Youn ger Dryas and the At lan tic pe riod. Un for tu -
nately we were not able to date the dis ap pear ance of the lake pre cisely. Or ganic sed i ments over lie coarse-grained river sands in bore hole BG-1 and gyttja in bore hole BG-2. Bore hole BG-1 dis plays a se quence of de tri tus gyttja (170–195 cm) that sharply trans forms into reed peat (90–170 cm), which is cov ered by al der peat (0–80 cm) (Fig. 2). The ra dio car bon age of the cen tral part of the gyttja layer is 11,086 cal. years BP (Ta ble 1). We as sume that the gyttja at the bot tom of bore hole BG-2 (165–200 cm) is of the same age as the gyttja in bore hole BG-1. On top of these de - pos its, there is a layer of reed peat (165–115 cm) with a grad ual tran si tion to al der peat (0–80 cm). Be tween 115–80 cm there is a grad ual tran si tion be tween reed peat and al der peat. The top layer of peat is strongly de com posed, most likely re sult ing from mire drain age in the 20th cen tury.
POLLEN ANALYSIS
In to tal 25 pol len sam ples were ana lysed in bore hole BG-1 (Fig. 3). Anal y ses were not car ried out for the top part of the bore hole, be cause of the strong de com po si tion of the peat.
Four pol len zones were iden ti fied by vi sual in spec tion, based on per cent ages of pol len at trib ut able to the most im por tant tree and her ba ceous plant spe cies (Ta ble 2 and Fig. 2).
SUCCESSION OF TERRESTRIAL VEGETATION
Wk-I (Ar te mi sia–Poaceae–Juniperus – LPAZ; Late Gla cial, 199–187 cm). At the close of Late Gla cial times, the ox bow lake was sur rounded by birch for ests with an ad mix ture of pine.
There were also shrubs of ju ni per (Juniperus) and wil low (Salix). The pres ence of open ar eas within the dry land scape is re flected by a con sid er able por tion of pol len of her ba ceous plants, in clud ing Ar te mi sia, and rep re sen ta tives of the Chenopodiaceae and Poaceae. In the lo cal veg e ta tion of ox - bow lakes Ranunculus acris-t was pres ent.
Wk-II (Betula–Pinus – LPAZ; Preboreal, 187–155 cm). A clear re gres sion of Juniperus communis, Poaceae, and Ar te mi - sia in di cates the Preboreal pe riod. Cli mate warm ing at the be - Fig. 3. Per cent age di a gram of non-pol len palynomorphs (NPPs) and pol len of aquatic and marsh plants of bore hole BG-1
gin ning of Ho lo cene times re sulted in rapid de vel op ment of birch for ests with ad mix tures of wil low, and sub se quently com - mu ni ties with a pre dom i nance of pine. Open ar eas were cov - ered by grass. An other sign of cli mate warm ing, among oth ers, is Hedera he lix.
Wk-III (Corylus–Pinus – LPAZ; Bo real, 155–99 cm). At ap - prox i mately 9899 cal. BP, for ests were dom i nated by pine, which grad u ally dis placed birch. Within the area stud ied ha zel (Corylus) ap peared, which pre ferred for est out skirts and more hu mid hab i tats (Fig. 2). Elm (Ulmus) and ash (Fraxinus) ap - peared. In some places in the wetlands Filipendula ulmaria was pres ent.
Wk-IV (Pinus–Alnus–Quercus – LPAZ; At lan tic, 99–55 cm).
Dur ing the At lan tic pe riod, we can see an in creas ing role of Pinus (Fig. 2). About 8700 cal. BP, in the up per part of the zone, a re cord of the dy namic ex pan sion of al der (Alnus) is also re - corded. In the for est, new tree spe cies (Tilia, Fraxinus, Quer - cus) are noted.
MICROSCOPIC REMAINS OF AQUATIC AND MARSH
Based on pol len anal y sis of aquatic and mire veg e ta tion and other microfossils (NPP – Non-Pol len Palynomorphs; Ta ble 3 and Fig. 3) we dis tin guished five zones il lus trat ing the evo lu tion of the aquatic -mire eco sys tem stud ied.
Wk-1 (199–195 cm, Late Gla cial). Open wa ter fa voured the de vel op ment of plank tonic or gan isms. Among the al gae these were spe cies of the ge nus Pediastrum. In wa ters macrophytes oc curred: Ceratophyllum sp., Nympheae alba and Myrio - phyllum spicatum. Around the lake rushes pre dom i nated with rep re sen ta tives of the Cyperaceae fam ily;
Wk-2 (195–163 cm, Preboreal). At ap prox i mately 11086 cal. BP, the wa ter level in the ox bow lake stud ied be - came lower, which is re flected in the higher fre quency of Filicales mono lete spores, Typha latifolia pol len and an ex pan - Fig. 4. Li thol ogy, ra dio car bon age and chem i cal com po si tion of sed i ments in BG-1 and BG-2 bore holes
and geo chem i cal zones dis tin guished based on the chem is try of the bore hole
sion of Filipendula ulmaria, which needs av er age July tem per a tures to be >16°C (Aar nes et al., 2012). Macro phytes such as Nymphaea alba and Cerato phyllum demersum grew in the lake (Fig.
3). An in creas ing fre quency of cyanobacteria Anaba ena is ob served while plank tonic spe cies of green al gae Pediastrum were de creas ing;
Wk-3 (163–123 cm, Bo real). In rush veg e ta - tion, Filicales monolete spores pre dom i nate.
There is higher fre quency of Typha latifolia and lower fre quency of macrophytes.
Wk-4 (123–83 cm, Bo real). In creas ing fre - quency of Cyperaceae pol lens as well as the pres ence of fun gal spores Sordaria and HdV - -200, which was syn chro nous with the de creas ing fre quency of Filicales monolete spo res, re corded at a depth of 111–95 cm.
Wk-5 (83–55 cm, At lan tic). In plant com mu ni - ties Filicales monolete spores are prev a lent, which is re flected in the higher fre quency of Thelypteris palustris spores (Fig. 4). As in the pre - vi ous phase, there was a high oc cur rence of Turbellaria.
GEOCHEMICAL ANALYSIS
Five geo chem i cal zones were dis tin guish ed in bore hole BG-1 us ing clus ter anal y sis (Fig. 4).
Zone I (195–170 cm) con sists of dark col oured de tri tus gyttja that ac cu mu lated dur ing the Youn - ger Dryas and Preboreal pe ri ods. The gyttja dis - plays in creas ing car bon ates (7.8–13.7%), TOC (63.82–257.59 g·kg–1) and N (3.91–20.96 g·kg–1).
The lake mud in bore hole BG-2 at the same depth in ter val con tains 34.17–106.48 g·kg–1 of TOC and 1.46–6.09 g·kg–1 of N. Both de posit types are poor in phos pho rus (0.37–1.18 g·kg–1 in gyttja and 0.28–0.48 g·kg–1 in mud) and rel a tively poor in K (3.10–4.28 g·kg–1 in gyttja and 1.88–5.77 g·kg–1 in mud), Fe (8.09–11.88 g·kg–1 in gyttja and 5.00–15.14 g·kg–1 in mud) and Al (9.64–12.85 g·kg–1 in gyttja and 5.33–14.82 g·kg–1 in mud).
Zone II (170–140 cm) rep re sents the ini tial phase of reed peat ac cu mu la tion dur ing Pre - boreal times and the be gin ning of Bo real times. It is char ac ter ized by de creas ing pH from 7.2 to 5.8 and de creas ing val ues of K (2.75–0.93 g·kg–1), Mg (3.21–1.37 g·kg–1) and Al (8.78–3.15 g·kg–1) (Fig. 4). TOC and N val ues in crease. The TOC:N ra tios are from 15:1 to 19:1.
Zone III (140–90 cm) rep re sents the sec ond phase of reed peat ac cu mu la tion. The peat is char ac ter ized by high amounts of TOC and N, which in the bore hole BG-1 are 463.63–549.46 g·
·kg–1 and 23.04–27.35 g·kg–1 re spec tively; and a sta ble pH from 5.6–5.8. The con tents of Na (0.31–0.40 g·kg–1), K (0.06–0.35 g·kg–1), Mg (0.67–0.94 g·kg–1), Al (0.26–1.50 g·kg–1), Mn (100.83–169.23 mg·kg–1), Cu (6.23–9.58 mg·kg–1) and Zn (4.28–15.89 mg·kg–1) are in gen eral low and show only small vari a tion in most cases.
Zone IV (90–35 cm) con sti tutes a suc ces sion of ter res trial veg e ta tion that grew dur ing the At lan - tic pe riod, which is re flected in the pres ence of al - Fig. 5. Rel a tive abun dance di a gram of subfossil cladoceran taxa
and di a gram of to tal num ber of Cladocera spec i mens, num ber of spe cies di ver sity in the sed i ments of the BG-1 pro file CZBG-1 – Cladocera as sem blage zones; other ex pla na tions as in Fig ure 4
der peat. The zone is char ac ter ized by large amounts of TOC (458.88–495.76 g·kg–1) and in creas ing con tents of N (22.09–29.62 g·kg–1) and P (0.30–0.83 g·kg–1), whereas the TOC:N and TOC:P ra tios de crease (from 22:1 to 16:1 and from 1648:1 to 531:1 re spec tively: Fig. 4).
The val ues of Na (0.28–0.35 g·kg–1), K (0.09–0.32 g·kg–1), Mg (0.78–1.04 g·kg–1), Al (1.02–4.12 g·kg–1), Mn (186.70–226.13 mg·kg–1) and Zn (2.48–12.26 mg·kg–1) are rel a tively sta ble over time and do not dif fer sig nif i cantly from those of zone III.
Zone V (35–0 cm) rep re sents stron - gly de com posed al der peat, of which the youn ger dates prob a bly re late to the end of the At lan tic pe riod. It is char ac ter ized by in creas ing pH (5.6–6.9) and amounts of P (0.75–3.49 g·kg–1), Na (0.38–0.98 g·kg–1), Mn (157.05–359.19 mg·kg–1), Cu (11.45–17.31 g·kg–1) and Zn (9.63–66.98 g·kg–1). The con tents of the re main ing el - e ments are rel a tively sta ble over time.
CLADOCERA ANALYSIS
25 spe cies of Cladocera were iden ti - fied in the bore holes in ves ti gated (22 spe - cies in bore hole BG-1 and 23 in bore hole BG-2), which be long to four fam i lies:
Chydo ridae, Sididae, Bosminidae and Daphnidae. Be tween these two pro files are mi nor dif fer ences in spe cies con tent (Figs. 5 and 6). In bore hole BG-1 Daph nia longispina-group and Camptocercus lillje - borgii were re corded, while these were ab sent in bore hole BG-2. On the other hand, in bore hole BG-2 Disparalona ros - tra ta, Peracantha truncata and Alona guttata were found, these not be ing iden - ti fied in bore hole BG-1. Among the spe - cies found in the sed i ments, the vast ma - jor ity was spe cies in hab it ing the lit to ral zone (22) liv ing on the bot tom and among the wa ter plants; only Bosmina longiro - stris, Daph nia longispina-group and Cerio daphnia sp. could be counted as pe - lagic spe cies. Three main zones, and two subzones of the Cladocera suc ces sion, were dis tin guished in each bore hole ba - sed on changes in the fre quency and num ber of spe cies pre served, and in di ca - tor spe cies. The zones are de scribed in Ta bles 4 and 5.
COMPARISON OF THE ZONES OF CLADOCERA ASSEMBLY DEVELOPMENT
IN PROFILES BG-1 AND BG-2
The be gin ning of Zone Ia con sti tutes a re cord of the ini tial stage of lake de vel - op ment (Figs. 5 and 6). This zone is char - ac ter ized by the high est num ber of spe - cies and in di vid u als of Cladocera but only
Fig. 6. Rel a tive abun dance di a gram of subfossil cladoceran taxa and di a gram of to tal num ber of Cladocera spec i mens, num ber of spe cies
di ver sity in the sed i ments of the BG-2 pro file
CZBG-2 – Cladocera as sem blage zones; other ex pla na tions as in Fig ure 4
in the deep est part of the ox bow lake that was better sup plied in the wa ter (BG1 – Fig. 5). A grad u ally in crease in the oc cur rence of spe cies in di cated a higher fer til ity of wa ter and abun dance of nu tri ents: Alona rectangula, Chydorus sphaericus (Alho nen, 1970) and the spe cies Pleuroxus uncinatus, Pleuroxus trigo - nellus and Dispara lona rostrata (White side, 1970). Re mains of spe cies char ac ter ized by high ther mal re quire ments were also found, e.g. Camptocercus rectirostris, Pleuroxus sp., Grapto - leberis testu dinaria (Poulsen, 1944). Spe cies liv ing in dif fer ent en vi ron ments were noted e.g., as so ci ated with bot tom sed i - ments (Alona quadrangularis) and aquatic plants (Acro perus harpae, Alona affinis, Alonella nana). A high num ber of in di vid u - als, which ex ceeded 4000 in di vid u als/cm3 of sed i ment, was noted. In the palaeochannel neck, where mud ac cu mu lated, the Cladocera spe cies com po si tion was sim i lar, but poorer with a much smaller num ber of in di vid u als, which did not ex ceed 800 in di vid u als/cm3 of sed i ment. In both bore holes sev eral in - creases in the num ber of the re mains of spe cies from the
open-wa ter group were iden ti fied e.g., Bosmina longirostris, Ceriodaphnia sp., and Daph nia longi spi na - group.
Zone Ib is de vel oped dif fer ently in the two bore holes. A vast im prove ment in the con di tions for de vel op ment took place in the part lo cated closer to the palaeochannel neck (BG-2 – Fig.
6). Greatly in creased num bers of spe cies were noted (to 22), the num ber of in di vid u als reach ing al most 3000 in di vid u als/cm3 of sed i ment. The main spe cies in di cated a higher fer til ity of the wa ters: Pleuroxus trigonellus, Chydorus sphaericus, Alona rectan gula, Bosmina longirostris, Graptole beris testudinaria, Disparalona rostrata and mesotrophic sta tus – Alonella exigua and Alonella trigonellus (Adamska and Mikulski, 1968). There was a rel a tively high num ber of spe cies as so ci ated with wa ter plants: Acroperus harpae, Alonella nana, Alonella excisa (Flössner, 1964). Cer tain spe cies were also pres ent that in di - cated a higher wa ter level: Bosmina longirostris and Cerio - daphnia sp.
T a b l e 1 Ra dio car bon dat ing re sults, bore hole BG-1
No. Depth
[cm] Dated
ma te rial No. of
dates Re sults of dates
in 14C yr. BP Cal i brated ages in cal. yr. BP
1. 104–106 peat MKL-2104 8100 ±90 8700
2. 147–149 peat MKL-2105 9050 ±100 9899
3. 177–179 de tri tus MKL-2106 9840 ±90 11086
T a b l e 2 De scrip tion of lo cal pol len as sem blage zones (LPAZ), pro file BG-1
Sym bol and name
of pol len zone Pe riod Depth
[cm] Main fea tures of pol len spec tra
Wk-I
(Ar te mi sia–Poaceae–Juniperus) Late Gla cial 199–187 High val ues of Ar te mi sia, Juniperus, and Poaceae pol len;
Ranunculus acris-t. oc curs reg u larly Wk-II
(Betula–Pinus) Preboreal 187–155 Very high pro por tion of Betula pol len; grad u ally in creas ing val ues of Pinus sylvestris type; Poaceae pol len oc curs reg u larly;
the first ap pear ance of Filipendula ulmaria pol len Wk-III
(Corylus–Pinus) Bo real 155–99 Rises in val ues of Pinus sylvestris type and de creas ing val ues of Betula and Poaceae; first ap pear ance of Corylus; Fraxinus and Ulmus
Filicales monolete spores are pres ent.
Wk-IV
(Pinus–Alnus–Quercus) At lan tic 99–55 Max i mum val ues of Pinus pol len; low val ues of Betula and Corylus;
first ap pear ance of Quercus and Tilia pol len; ris ing val ues of Alnus
T a b l e 3 De scrip tion of as sem blage zones dis tin guished in pro file BG-1, on the ba sis of pol len taxa of aquatic,
mire plants and non-pol len palynomorphs (NPPs) Sym bol and name
of zone
Sym bol and name RPAZ
Depth
[cm] Main fea tures of pol len spec tra
Wk-1 Late Gla cial 199–195 High pro por tion of Pediastrum; sin gle pol len grains of Myriophyllum spicatum; de creas ing curve of Cyperaceae
Wk-2 YD/Preboreal 195–163 Max i mum of Ceratophyllum-haris, Nympheae alba, Equisetum; de creas ing curve of Pediastrum sp.; rise of Filicales monolete spores Wk-3 Bo real 163–123 Sin gle pol len grains of Typha latifolia and sin gle spores of fungi HdV-200
and Persicispora; ris ing curve of Filicales monolete spores Wk-4 Bo real 123–83 Ris ing curve of Cyperaceae and de creas ing curve of Filicales monolete;
high pro por tion of HdV-200 and Sordaria; fre quent oc cur rence of Nymphaea alba
Wk-5 At lan tic 83–55 Max i mum of Filicales monolete spores; low pro por tion of Cyperaceae;
rise of Turbellaria curve
In the mid dle part of the lake (BG-1 – Fig. 5) there was noted a de cline in the num ber of Cladocera re mains. Ini tially, the num ber of spe cies was 15, then de creased to 9. Alonella excisa pre dom i nated: a spe cies that dwells on aquatic plants – reeds and watermilfoil (Myriophyllum) and Pleuroxus trigo - nellus – liv ing on the bot tom, sim i lar to A. excisa, show ing a high tol er ance to low pH (Krause-Dellin and Steinberg, 1986).
High oc cur rences were also noted for two spe cies re sis tant to en vi ron men tal stress as so ci ated with a low er ing of wa ter level:
Chydorus sphaericus and Alona rectangula. At the end of Zone Ia there ap peared the spe cies Alona intermedia, con tin - u ing its oc cur rence into Zone Ib. There was a lack of spe cies typ i cal of open wa ter.
Zone Ib of bore hole BG-2 is co in ci dent with Zone Ib in the BG-1 bore hole, with a com pa ra ble num ber of spe cies, but a smaller num ber of in di vid u als of Cladocera (Figs. 5 and 6). In both parts of the lake there oc curred spe cies as so ci ated with
the bot tom and with aquatic plants. Spe cies from the open wa - ter zone are found only in Zone Ia of bore hole BG-1 and in subzones Ia and Ib of bore hole BG-2. In both parts of the lake there oc curred spe cies as so ci ated with bot tom and aquatic plants.
Re mains of Cladocera prob a bly ac cu mu lated in both pro - files at the same time but in dif fer ent hy dro log i cal sit u a tions, af - fected by the slightly dif fer ent com po si tions and num bers of Cladocera. In both pro files at the end of Zone Ib there was a re - treat from the lake of al most all species of Cladocera.
Zone II was as so ci ated with a lack of iden ti fied Cladocera re mains in bore hole BG-1, rep re sented only by a few ephippia, i.e. the rest ing eggs of Cladocera. In the BG-2 bore hole (Fig. 6) this zone is char ac ter ized by the pres ence of only one spe cies (Chydorus sphaericus), of very broad en vi ron men tal tol er ance.
At the be gin ning of this zone, Alonella excisa oc curred in the lake, dwell ing among wa ter plants and also re sis tant to low pH, T a b l e 4 De scrip tion of Cladocera as sem blage zones (CZBG-1) dis tin guished in pro file BG-1
Sym bol Depth
[cm] Main fea tures of Cladocera spec tra
CZBG-1 Ia 199–137
The best de vel op ment con di tions for the zoo plank ton. 18 spe cies were iden ti fied (Fig. 5). The to tal num ber of in di - vid u als of Cladocera re corded is around 4080 spec i mens in 1 cm3 of sed i ment. In this zone there was the max i - mum num ber of spe cies and in di vid u als. Pre dom i nant spe cies: Alona rectangula (max. 33%), Alonella nana (max. 33%), Chydorus sphaericus (max. 22%), Alona affinis (max. 17%), Acroperus harpae (max. 15%). Spe cies from dif fer ent hab i tat groups oc curred. High oc cur rence of spe cies from the open wa ter zone (max. 13% – all spe - cies). High est in dex of biodiversity (1.0) in the lake (Fig. 5).
CZBG-1 Ib 137–73
De te ri o ra tion of liv ing con di tions in the lake. De creas ing abun dance of spec i mens (max. 1950 sp./cm3 of sed i - ment) and spe cies (15). Main spe cies are con nected with wa ter plants: Alona rectangula (31%), Alona intermedia (max. 24%), Camptocercus rectirostris (max. 12%) and dom i nant spe cies Alonella excisa (max. 69%) which is also tol er ant of lower pH. High abun dance of one spe cies Pleuroxus uncinatus (max. 38%) con nected with sed i - ment and ubiq ui tous Chydorus sphaericus (max. 20%). Lack of spe cies from open wa ter zone. Biodiversity in dex was high at the be gin ning of the zone and then dropped to 0.6.
CZBG-1 II 73–41 Scar city of Cladocera re mains in the sed i ment; only a few Cladocera ephippia were iden ti fied.
CZBG-1 III 41–1
Slight im prove ment of con di tions for zoo plank ton de vel op ment. In crease in fre quency of spe cies to 9 (Fig. 5). In - crease in oc cur rence of spec i mens: >750 in di vid u als of Cladocera in 1 cm3 of sed i ment. Main spe cies are con - nected with wa ter plants: Alonella excisa (max. 45%), Alona rectangula (18%) Alona intermedia (max. 17%) and Chydorus sphaericus (52–100% at the end).
T a b l e 5 De scrip tion of Cladocera as sem blage zones (CZBG-2) dis tin guished in pro file BG-2
Sym bol Depth
[cm] Main fea tures of Cladocera spec tra
CZBG-2 Ia 199–175
In this zone the pres ence of 13 spe cies of Cladocera was de ter mined (Fig. 6). The to tal fre quency amounted to 525–800 sp./cm3 of sed i ment. Spe cies from all hab i tat groups were found. Chydorus sphaericus pre dom i nated, with max. 52% con tri bu tion. There were also nu mer ous spec i mens of spe cies such as: Pleuroxus uncinatus (max. 29%), Acroperus harpae (max. 19%), Alona affinis and Bosmina longirostris (max. 13%). The biodiversity in dex was 0.66 at the be gin ning and in creased to 0.99.
CZBG-2 Ib 175–97
The best zone for Cladocera de vel op ment. 22 spe cies were iden ti fied. To tal num ber of in di vid u als of Cladocera in the sed i ments was above 2850 sp./cm3 of sed i ment. In this zone there was the max i mum num ber of spe cies and in di vid u als. Pre dom i nant spe cies: ubiq ui tous Chydorus sphaericus (max. 52%), Pleuroxus uncinatus (max.
29%), Acroperus harpae (max. 19%), Alona affinis (max. 13%) and Bosmina longirostris (max. 13%). The main spe cies were con nected with sed i ments and as so ci ated with macrophytes. Spe cies from the open wa ter zone were found. The biodiversity in dex was about 1.0.
CZBG-2 II 97–49 Sig nif i cant de te ri o ra tion of liv ing con di tions in the lake. De creas ing abun dance of spec i mens (<250 sp./g of sed i - ment) and spe cies (3). Pre dom i nant Graptoleberis testudinaria (max. 49%) and widely tol er ant Chydorus sphaericus (max. 51%). Lack of Cladocera from the open wa ter zone.
CZBG-2 III 49–9
Im prove ment of de vel op ment con di tions. In crease in the fre quency of spe cies to 11 and in the oc cur rence of spec i mens: >1600 sp./cm3 of sed i ments (Fig. 6). Pre dom i nant spe cies con nected with wa ter plants: Alonella excisa (max. 33%), Alona guttata (33%) Alona affinis (max. 13%), Graptoleberis testudinaria (max. 14%), Pleuroxus trigonellus (max. 14%) – con nected with sed i ment and Chydorus sphaericus (max. 33%). Lack of Cladocera from the open wa ter zone.
sim i larly to Ch. sphaericus (Krause-Dellin and Steinberg, 1986). In the mid dle part of this zone there ap peared Graptole - beris testudinaria (Rybak and B³êdzki, 2010).
Zone III rep re sents im prov ing con di tions of Cladocera de - vel op ment, which is re flected in the in creas ing num ber of spe - cies to nine in bore hole BG-1 (Fig. 5) and to 10 in bore hole BG-2 (Fig. 6), also as so ci ated with an in crease in the num ber of in di vid u als, up to 750 and 1600 re spec tively. The pre dom i nant spe cies lived on sub merged plants and sed i ments. The most fre quent were: Ch. sphaericus, Alona excisa, Alona rectangula and Pleuroxus sp. These spe cies are re sis tant to low pH, low wa ter level and may be found in peat bogs and in dystrophic lakes (Duigan, 1992; Rybak and B³êdzki, 2010).
DISCUSSION
RECONSTRUCTION OF OXBOW LAKE DEVELOPMENT IN THE WKRA RIVER VALLEY NEAR BELAWY GO£USKIE
River me an ders pro vide ev i dence of ma jor en vi ron men tal dy nam ics of river val ley geosystems. Spe cific aquatic-mire eco - sys tems de velop in ox bow lakes af ter cut-off from the river, char ac ter ized over time by vari able in ten sity of river and ground wa ter sup ply, these be ing strongly con trolled by cli ma tic con di tions. Pe ri odic sup ply of river wa ter, car ry ing a va ri ety of sus pended and dis solved sub stances, plays an im por tant role in the func tion ing of these eco sys tems, par tic u larly strongly in - flu enc ing plank tonic or gan isms and aquatic plant com mu ni ties.
Dur ing their de vel op ment, ox bow lakes grad u ally fill with strat i - fied min eral and or ganic sed i ments of var ied or i gins and prop - er ties, which con sti tute a geo chem i cal and palaeo geo graphi cal re cord of the ox bow lake and its catch ment evo lu tion (Paw³o -
wski et al., 2015a). How ever, they have rarely been used for pur poses of palaeoenvironmental re con struc tions.
The use ful ness of palaeo-ox bow lake sed i ments for palaeo - environmental re con struc tions sug gested by Paw³owski et al.
(2015a) is con firmed by our stud ies in the val ley of the Wkra River. Based on geo chem i cal, pol len and fos sil Clado cera anal - y sis, sup ported by ra dio car bon dat ing, we re con structed the pe - riod of ox bow lake for ma tion and its cut-off from the river, as well as the tem po ral dy nam ics of eco log i cal con di tions in the res er - voir and its catch ment. The main stages of evo lu tion re con stru - cted are de scribed be low.
Youn ger Dryas (LPAZ-Wk-I; zone Wk-1, 2; geo chem i cal zone I; Cladocera – be gin ning of Ia CZBG-1 and Ia CZBG-2, Fig. 7). Steppe veg e ta tion, in di cat ing dry and cold cli mate con - di tions, prob a bly pre dom i nated in the vi cin ity of the newly formed ox bow lake at the close of the Youn ger Dryas. In the open for ests birch (Betula) and pine (Pinus) pre dom i nated, whereas grasses (Poaceae), sage brush (Ar te mi sia) and shrubs, par tic u larly ju ni per (Juniperus) cov ered open spaces. In the deeper zone of the lake (bore hole BG-1) de tri tus gyttja was ac cu mu lat ing, which con tained or ganic mat ter and car bon ates that in creased with time. Prob a bly dur ing the same pe riod in the neck of ox bow lake, fine-grained min eral sed i ments were ac cu - mu lat ing, car ried in by river wa ters (Fig. 7). The spa tial vari abil - ity in sed i ment prop er ties in di cates var ied eco log i cal con di tions in the in di vid ual zones of the lake in the ini tial phase of its de vel - op ment, and high lights the im por tance of spa tial stud ies in palaeoenvironmental re con struc tions. A con sid er able depth of the lake is shown by the pres ence of Cladocera taxa typ i cal of open wa ters, such as Bosmina longirostris and Ceriodaphnia sp. Fre quently oc cur ring spe cies of Pediastrum are rel a tively abun dant in ni tro gen-rich sed i ments, in di cat ing the pres ence of a mod er ately deep, eutrophic wa ter body, which was con tin u - ously sup plied with biogenic sub stances from river wa ters.
Fig. 7. Com par i son of the palaeohydrological data for the pro file BG-1 based on pol len, geo chem i cal and Cladocera anal y sis
Such con di tions fa voured the de vel op ment of Cladocera, which is sup ported by the in creas ing fre quency of dif fer ent eco log i cal groups of these or gan isms, es pe cially of spe cies with high trophic re quire ments (viz. Alona rectangula, Chydorus sphaeri - cus, Pleuroxus trigonellus). Spe cies were also ob ser ved with higher ther mal re quire ments – Pleuroxus sp., Grapto le beris testudinaria, Camptocercus rectirostris, which be long to south tem per ate spe cies (Hofmann, 2000; Szeroczyñska and Zawisza, 2007). These spe cies have also been no ticed in other wa ter bod ies from the Youn ger Dryas, in clud ing in the palaeo - -ox bow lakes of Korzeñ and Kolonia Bechcice and Goœci¹¿
Lake (Szeroczyñska and Zawisza, 2007; Paw³owski, 2012).
The pres ence of thermophilic spe cies (Typha latifolia) is prob a - bly as so ci ated with pe ri odic in creases in tem per a ture dur ing the Youn ger Dryas, when min i mum av er age July tem per a tures in - creased – to 13°C (Ralska-Jasiewiczowa et al., 2004) while the pres ence of the hydrophyte Myriophyllum spicatum in di cates that the tem per a ture was above 10°C (Bos et al., 2007).
Preboreal (LPAZ – Wk-II; zone Wk-2, 3; geo chem i cal zone II; Cladocera – mid part CZGB-1 Ia, be gin ning CZGB-2 Ib). A gen eral ten dency to cli mate warm ing at the close of the Youn - ger Dryas and be gin ning of Ho lo cene times in the area of Cen - tral Eu rope, and re lated de crease of wa ter level in in land res er - voirs (Magny and Bégeot, 2004; Bos et al., 2007), is sup ported by the ap pear ance of Ceratophyllum demersum, Nymphaea alba and Typha latifolia in aquatic and rush plant as sem blages.
The rel a tively in tense de vel op ment of these spe cies sug gests av er age July tem per a tures of 13–15°C (Isarin and Bohncke, 1999; Ralska-Jasiewiczowa et al., 2004; Bos et al., 2007), which is also shown by the pres ence of thermophilic Cladocera (Pleuroxus sp., Camptocercus rectirostris). A con sid er able de - crease of wa ter level dur ing this pe riod was no ticed also by Karasiewicz et al. (2014) in Retno Lake and Zawiska et al.
(2015) in £ukie Lake. This in crease of tem per a tures re sulted in an in creas ing fre quency of both in di vid u als and spe cies of Cladocera. Groups of spe cies were rep re sented that are as so - ci ated with aquatic plants and bot tom sed i ments. The high est fre quen cies in volved also spe cies of higher trophic re quire - ments: Alona rectangula, Ch. sphaericus, Pleuroxus trigo - nellus, and Anabaena pre dom i nated within al gae. This was prob a bly the pe riod most fa vour able for Cladocera de vel op - ment in the lake stud ied. Such a sit u a tion was no ticed in most of the lakes in ves ti gated in Po land (e.g., Milecka and Szero - czyñska, 2005; Szeroczyñska, 2006). In the sec ond half of the Preboreal pe riod, a low er ing of wa ter level took place in the ox - bow lake, which is re flected in the change of sed i ment type. Ac - cu mu la tion of or ganic-rich reed peat be gan. At the same time, the fre quency of Cladocera spe cies typ i cal of open wa ters, in - clud ing Bosmina longirostris and Alona quadrangularis, de - creased. In the sed i ments there ap peared fun gal spores of Persicispora sp. ge nus and HdV-200, which may be pres ent in de cay ing wood and re mains of plants as so ci ated with tem po - rarily dry ing wa ter bod ies (Van Geel et al., 1989; Kuhry, 1997;
Aptroot and Van Geel, 2006). The ver ti cal vari abil ity in the chem is try of the de pos its from this pe riod (e.g., in creas ing con - tent of ni tro gen and phos pho rus, de creas ing amounts of com - po nents as so ci ated with the min eral phase, in creas ing val ues of TOC:N and TOC:P) sug gests a grad ual de te ri o ra tion of trophic con di tions (Fig. 7), which is prob a bly due to the de creas - ing role of river wa ters in sup ply ing the lake (re flected also in the de creas ing trends in K and Al con tent).
Bo real (LPAZ – Wk-III; zone Wk-3, 4; geo chem i cal zone II; Cladocera – end of Ia CZBG-1, mid part Ib CZBG-2). An ex - pan sion of ha zel (Corylus avellana) at ap prox i mately 9899 cal.
BP is ob served. This is typ i cal for many ar eas in Eu rope, dated
to about 10,500 cal BP, and cli mate warm ing is con sid ered as the ma jor fac tor de ter min ing the ex pan sion of this spe cies (Finsin ger et al., 2006; Giesecke et al., 2011). At the be gin ning of the Bo real pe riod, for ests were dom i nated by pine (Pinus), which grad u ally dis placed birch (Betula). In ri par ian for ests there ap peared elm (Ulmus) and wil low (Salix), which oc curred within wetlands. The pres ence of Filipendula ulmaria sug gests a pro gres sive warm ing of cli mate (Aarnes et al., 2012). Con di - tions were highly fa vour able for zoo plank ton de vel op ment. In the ox bow lake stud ied, we can see the re ap pear ance of spe - cies typ i cal of open wa ters, which in di cates an in crease in wa - ter level. At the be gin ning of this pe riod, the high fre quency of spe cies in hab it ing the area among aquatic plants Alonella nana, Acroperus harpae, Alona affinis, and hav ing higher ther - mal and trophic re quire ments, per sists. The in creas ing fre - quency of Cyperaceae pol len, as well as of fun gal spores of Sordaria and HdV-200 in sed i ments at depths of 95–111 cm, in di cates a drier pe riod. This is also shown by the de crease of Filicales mono lete spore fre quency and a con sid er able de - crease in Cladocera spe cies di ver sity, es pe cially in the lack of spe cies as so ci ated with open wa ters. In par al lel, we can see an in creas ing fre quency of spe cies re sis tant to low pH (Alo - nella excisa). In this pe riod, the ox bow lake was com pletely cut-off from the river, which is also shown by the large and uni - form amounts of or ganic mat ter in the reed peat, as well as in the low est con cen tra tions of Al, K and Mn within the bore holes.
The low con tents of these el e ments sug gest a low in ten sity of me chan i cal de nu da tion in the catch ment (Borówka, 2007; Fig.
7). The rel a tively high con tents of Ca show the im por tant role of ground wa ter in lake sup ply. The pro gres sive in creases in TOC:N and TOC:P ra tios in di cate a de te ri o ra tion of trophic con di tions. The dry ing of cli mate from this pe riod is also re - flected in iso to pic data from Charzykowskie Lake (Miros³aw - -Grabowska and Zawisza, 2014).
At lan tic pe riod (LPAZ – Wk-IV; zone Wk-5; geo chem i cal zone 4; Cladocera – CZGB-1 – end Ib,II, III; CZGB-2 – II, III).
From the be gin ning of the At lan tic pe riod, about 8,700 cal. BP, an ex pan sion of al der (Alnus) is ob served within the lake catch - ment. This pro cess had a syn chro nous char ac ter in the Bal tic Sea re gion due to fa vour able hab i tat con di tions within post - glacial ar eas (Giesecke et al., 2011). New tree spe cies (Tilia, Fraxinus, Quercus) ap peared, which in di cate the im prove ment of cli ma tic and edaphic con di tions. A rad i cal change in eco log i - cal con di tions of the lake took place in this phase, in clud ing a con sid er able de cline in wa ter level. At the be gin ning of this pe - riod, the lake was sur rounded by a line of rush veg e ta tion with Thelypteris palustris. Peat from this pe riod is char ac ter ized by only a slightly lower con tent of or ganic mat ter com pared to the pre vi ous one. In creas ing con tents of ni tro gen and phos pho rus and nar row ing val ues of the TOC:N and TOC:P ra tios are in di - ces of grow ing trophic char ac ter of the lake. Ground wa ter still played a pre dom i nant role in its sup ply. At the be gin ning of the At lan tic pe riod, the res er voir was in hab ited by the spe cies of Cladocera most re sis tant to en vi ron men tal stress, in clud ing Chydorus sphaericus, Alonella excisa, Alona rectagula and Pleuroxus sp. (Duigan, 1992); how ever, in the next pe riod (CZBG-1 III, CZBG-2 III), al most ev ery one of these re treated.
This left only the most tol er ant spe cies: Chydorus sphaericus.
The pres ence of ephippia (rest ing eggs of Cladocera) also sug - gests the un fa vor able en vi ron men tal con di tions (Sarmaja - -Karjonen, 2003). At the end of the lake de vel op ment, an in - crease in Cladocera fre quency took place sev eral times, es pe - cially for spe cies with wide en vi ron men tal tol er ance, which could in di cate pe ri odic sup ply of river wa ter. Dur ing this pe riod, lake wa ters were eutrophic in their char ac ter, which is sup -
ported by the pres ence of Turbellaria. The data ob tained do not al low us pre cisely to de lin eate the tim ing of lake dis ap pear ance.
It prob a bly took place at the close of the At lan tic pe riod.
Geo chem i cal fea tures of the top of sed i ments have been strongly mod i fied by hu man ac tiv ity dur ing the last few cen tu - ries. Strong de com po si tion of peat mass as a re sult of drain age works and Wkra River chan nel straight en ing in the 20th cen - tury, and el e vated con cen tra tions of phos pho rus and heavy met als, are the most per cep ti ble signs of this impact.
CONCLUSIONS
Our study shows the im por tance of palaeo-ox bow lakes as ar chives of river val ley en vi ron men tal evo lu tion. Based on geo - chem i cal and palaeo geo graphi cal anal y ses, we have re con - structed some im por tant as pects of the Late Pleis to cene and Ho lo cene evo lu tion of the Wkra River Val ley en vi ron ment within the area stud ied. The re sults il lus trat ing the spatio temporal dy - nam ics of the aquatic-mire eco sys tem pro vide in for ma tion to help re con struct the pe riod of ox bow lake for ma tion and its cut-off from the river, hy dro log i cal con di tions within the catch - ment, the in ten sity of me chan i cal and chem i cal de nu da tion, as well as some as pects of the in flu ence of hu man ac tiv ity (Fig. 7).
We dis tin guished dry and wet phases in lake de vel op ment and made an at tempt to cor re late these with other stands from Po - land, and widely dis cussed ep i sodes of cli ma tic change. The most im por tant find ings aris ing from our study are the fol low ing:
– Cut-off from the river of the me an der took place dur ing the Youn ger Dryas. How ever, the newly formed ox bow lake was sup plied by river wa ter un til the be gin ning of Bo real times.
– Wa ter level fluc tu a tions in the lake re lated to gen eral ten - den cies ob served in river val leys on the Pol ish Plain. Dur - ing the Youn ger Dryas and the be gin ning of Ho lo cene time, there oc curred pe ri ods of high wa ter lev els, whereas at the close of Bo real time and the be gin ning of At lan tic time, we no ticed a de crease in wa ter level. The re cord of a dry phase from about 8100 cal BP is very clear.
– The lake in its ini tial phase of de vel op ment was sup plied mainly by river wa ter, the im por tance of which de -
creased over time. From the be gin ning of Bo real time, ground wa ter played the ma jor role in res er voir sup ply.
This is con firmed both by the ver ti cal vari abil ity of geo - chem i cal fea tures of the sed i ments and the subfossil Cladorera and pol len con tained within them.
– The in ten sity of me chan i cal de nu da tion in the lake catch ment was in gen eral low, which is sup ported by the sig nif i cant con tent of or ganic mat ter and lack of Al, K and Mn in the sed i ment bore holes.
– The ver ti cally vari able con tents of TOC, N as well as TOC:N and TOC:P ra tios in the sed i ment bore holes, re - flect the tem po ral vari abil ity in the trophic con di tions of the lake, which were strongly af fected by the source of wa ter sup ply. In the ini tial phase of de vel op ment, it was a eutrophic res er voir sup plied by river wa ter rich in biogenic sub stances. The in creas ing role of ground wa - ter is re flected in de creas ing trophic char ac ter.
– The dis ap pear ance of the lake took place prob a bly at the close of the At lan tic pe riod; how ever, we can not give the ex act date.
– The top layer of sed i ments con sti tutes a re cord of hu - man ac tiv ity dur ing the most re cent times, es pe cially the 20th cen tury. This is re flected in the strong de com po si - tion of peat mass due to drain age and river chan nel straight en ing in the 20th cen tury, and in the el e vated con cen tra tion of phos pho rus and heavy met als.
– The re sults of our study high light the im por tance of spa - tial anal y sis in en vi ron men tal re con struc tions based on sed i ment bore hole. Anal y sis of bore holes taken from sev eral points al lows us to cap ture the spa tial vari abil ity of the pro cesses and their in ten sity oc cur ring in palaeo - lakes. Such an ap proach is par tic u larly im por tant in stud - ies of palaeo-ox bow lakes.
Ac knowl edge ments. The stud ies were sup ported by the stat u tory of the In sti tute of Ge og ra phy and Re gional Stud ies, Pom er a nian Uni ver sity in S³upsk, pro ject no. 13/1/15. We would like to ex press our spe cial ap pre ci a tion to Prof. K. Szero - czyñska (Pol ish Acad emy of Sci ences, War saw) and the anon - y mous re viewer who helped to im prove our manu script.
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