Lake Compartments Research Articles

Inorganic component in oak waterlogged archaeological wood and volcanic lake compartments

Abstract. Waterlogged archaeological wood (WAW) is a rare and precious organic material that can hold outstanding cultural values. In order to protect WAW for the next generations, this material must be accurately characterised to set its proper conservation, storage and exhibition conditions in museum environments. In this study, the mineral content found in WAW retrieved in a volcanic lake was investigated by analysing wood ash through scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (SEM-EDS). This micro-destructive approach was coupled with morphological studies carried out through optical microscopy. SEM-EDS was also performed on the WAW and surrounding sediment to study the possible relationship between the mineral composition and the wood degradation state. The analysis revealed that calcium was the most abundant element in all poles, with weight percentages ranging between 24 % and 42 %. This element was more represented in heartwood (HW) than sapwood (SW). In sapwood the second most abundant element was arsenic. Sulfur, iron and potassium were present in all the analysed samples as well. Arsenic was also detected in the sediments; it was particularly concentrated in the samples taken near archaeological wood. The presence of this element can be linked to the volcanic origin of the lake, and its high concentration points to bioaccumulation processes induced by bacteria (erosion bacteria and sulfate-reducing bacteria) and biogeochemical processes favouring precipitation of insoluble compounds. The present work is the first investigation of the mineral content in archaeological wood establishing a contingent relationship with the surrounding volcanic lake sediments.

Effects of DOC addition from different sources on phytoplankton community in a temperate eutrophic lake: An experimental study exploring lake compartments

A mesocosm experiment was conducted in a temperate eutrophic lake with the hypotheses: 1) the addition of a labile form of DOC would trigger a more pronounced response in phytoplankton biomass and composition compared with a non-labile form; 2) DOC addition would increase phytoplankton biomass by co-inserting organic nutrients for phytoplankton growth; 3) DOC addition would change phytoplankton composition, in particular towards mixotrophic taxa due to higher DOC availability; and that 4) there would be differences in phytoplankton responses to DOC addition, depending on whether sediment was included or not. We used two types of mesocosms: pelagic mesocosms with closed bottom, and benthic mesocosms open to the sediment. The experiment ran for 29 days in total. The DOC addition occurred once, at Day 1. Besides the control, there were two treatments: HuminFeed® (non-labile DOC) at a concentration of 2 mg L−1, and a combination of 2 mg L−1 HuminFeed® and 2 mg L−1 DOC from alder leaf leachate (labile). Responses were detected only in the treatment with alder leaf extract. Ecosystem processes responded immediately to DOC addition, with the fall in dissolved oxygen and pH indicating an increase in respiration, relative to primary production (Day 2). In contrast, there was a delay of a few days in structural responses in the phytoplankton community (Day 6). Phytoplankton biomass increased after DOC addition, probably boosted by the phosphorus released from alder leaf extract. Changes in phytoplankton composition towards mixotrophic taxa were not as strong as changes in biomass, and happened only in the pelagic mesocosms. With the DOC addition, diatoms prevailed in benthic mesocosms, while the contribution of colonial buoyant cyanobacteria increased in the pelagic ones. This study points towards the necessity to look in greater detail at specific responses of phytoplankton to DOC concentration increases considering lake-habitat and sediment influence.

High concentrations of dissolved biogenic methane associated with cyanobacterial blooms in East African lake surface water

The contribution of oxic methane production to greenhouse gas emissions from lakes is globally relevant, yet uncertainties remain about the levels up to which methanogenesis can counterbalance methanotrophy by leading to CH4 oversaturation in productive surface waters. Here, we explored the biogeochemical and microbial community variation patterns in a meromictic soda lake, in the East African Rift Valley (Kenya), showing an extraordinarily high concentration of methane in oxic waters (up to 156 µmol L−1). Vertical profiles of dissolved gases and their isotopic signature indicated a biogenic origin of CH4. A bloom of Oxyphotobacteria co-occurred with abundant hydrogenotrophic and acetoclastic methanogens, mostly found within suspended aggregates promoting the interactions between Bacteria, Cyanobacteria, and Archaea. Moreover, aggregate sedimentation appeared critical in connecting the lake compartments through biomass and organic matter transfer. Our findings provide insights into understanding how hydrogeochemical features of a meromictic soda lake, the origin of carbon sources, and the microbial community profiles, could promote methane oversaturation and production up to exceptionally high rates.

Organic micropollutants in water and sediment from Lake Mälaren, Sweden

The occurrence and distribution of 111 organic micropollutants (OMPs) were evaluated in water and sediment samples from Lake Mälaren, Sweden, using a liquid chromatography-tandem mass spectrometry method. The partitioning of contaminants between lake compartments was estimated using solid water distribution coefficients (Kd) and organic carbon-water partitioning coefficients (KOC). In total, 30 and 24 OMPs were detected in lake water and sediment, respectively. Concentrations ranged from low ng/L to 89 ng/L (lamotrigine) in lake water and from low ng/g dry weight (dw) to 28 ng/g dw (citalopram) in sediment. Carbamazepine, lamotrigine, caffeine, and tolyltriazole were the dominant compounds in Lake Mälaren samples (both water and sediment). Seventeen OMPs were detected in both water and sediment samples, including carbamazepine, DEET, tolyltriazole, bicalutamide, caffeine, lamotrigine, and cetirizine. Log Kd values varied between 0.84 for lamotrigine and 4.4 for citalopram, while log KOC values varied between 2.1 for lamotrigine and 5.9 for citalopram. These results indicate that sorption to sediment plays a minor role in removal of all OMPs analyzed in the aqueous phase except for citalopram and cetirizine, which showed high sorption potential. The environmental risks of OMPs were assessed based on the RQ values. The worst-case scenario for environmental risk assessment was conducted using the maximum measured environment concentration. For most of the target OMPs, including tolyltriazole, bicalutamide, fexofenadine, oxazepam, cetirizine, and diclofenac, the RQ values were below 0.01, indicating low or no risk to lake ecosystems.

Beyond Selenium: Coal Combustion Residuals Lead to Multielement Enrichment in Receiving Lake Food Webs.

Effluents from coal-fired power plant ash ponds are a major source of environmental contamination, annually loading more than a million metric tons of pollutants to aquatic ecosystems in the United States alone. Though this waste stream is characterized by elevated concentrations of numerous inorganic constituents, decades of previous research effort have focused on the ecotoxicological consequences of a single stressor: selenium. In this study, we compared concentrations of 10 trace elements among three North Carolina reservoirs with varying burdens following decades of coal combustion residual (CCR) inputs. Along this pollution gradient, we examined (1) environmental compartment-specific trace element enrichment relative to reference lake levels and (2) differences in CCR accumulation patterns among abiotic and biotic compartments. We report significant multivariate differences between CCR-receiving and reference lakes for surface water, pore water, sediment, and fish tissues as well as differences in CCR accumulation among North Carolina resident fish species. Multiple-element enrichment across receiving lake compartments additionally highlighted that CCR pollution is a mixtures contamination issue. Our results inform the ongoing discussion about effective regulation of impaired water bodies and identify important questions that might guide the monitoring of these systems as they recover.

Bacterial diversity in the water column of meromictic Lake Cadagno and evidence for seasonal dynamics

The meromictic Lake Cadagno is characterized by a compact chemocline with high concentrations of anoxygenic phototrophic purple and green sulfur bacteria. However, a complete picture of the bacterial diversity, and in particular of effects of seasonality and compartmentalization is missing. To characterize bacterial communities and elucidate relationships between them and their surrounding environment high-throughput 16S rRNA gene pyrosequencing was conducted. Proteobacteria, Chlorobi, Verrucomicrobia, and Actinobacteria were the dominant groups in Lake Cadagno water column. Moreover, bacterial interaction within the chemocline and between oxic and anoxic lake compartments were investigated through fluorescence in situ hybridization (FISH) and flow cytometry (FCM). The different populations of purple sulfur bacteria (PSB) and green sulfur bacteria (GSB) in the chemocline indicate seasonal dynamics of phototrophic sulfur bacteria composition. Interestingly, an exceptional bloom of a cyanobacteria population in the oxic-anoxic transition zone affected the common spatial distribution of phototrophic sulfur bacteria with consequence on chemocline location and water column stability. Our study suggests that both bacterial interactions between different lake compartments and within the chemocline can be a dynamic process influencing the stratification structure of Lake Cadagno water column.

Beach ridge sets reflect the late Holocene evolution of the St Lucia estuarine lake system, South Africa

Sets of sandy beach ridges and intervening swales define shoreline sections of the shallow St Lucia wetland system within the iSimangaliso Wetland Park, World Heritage Site in northern KwaZulu-Natal province, South Africa. The sets comprise 3–10 beach ridges, the most prominent being 80–150 m wide and rising 0.5–2 m above the adjacent swales. The highest beach ridge crests elevated 3.2–4.6 m above mean sea level are furthest from the present shoreline and the lowest ridges, rising about 0.5 m above the swales, occur closest to the present mean lake level shoreline. This investigation assesses the genesis of the sandy beach ridges on five strand plain remnants within the estuarine lake. The ridges were topographically surveyed and their ages estimated using optically stimulated luminescence (OSL) dating. OSL dating reveals that the oldest beach ridge formed around 6240 years ago, with a sequence of beach ridges having accumulated during the period ~4000 to 1500 years ago reflecting coeval accretion around the lake. The present mean lake level shoreline was only reached within the past ~600 years. Sedimentation changed from marine-dominated to lacustrine deposition in the estuary during the period of beach ridge accretion. The dated beach ridges, supported by new radiocarbon dates of fixed biological indicators from Holocene intertidal zone settings on the coast, are used with published sea-level curves to set the context of periodic beach ridge accretion in the marine-linked estuarine lake. The dated ridges suggest that episodic regression of the estuarine lake shoreline occurred after a mid-Holocene sea-level highstand. The record of sea-level change is also reflected by the long-term natural shrinking and shallowing of the proto-St Lucia lagoon/estuarine lake in the context of reduced marine influences due to closure of former marine channels by progressive barrier dune accretion. The sequence of coeval beach ridges reflects a pulsed lowering of relative sea-level in the shallowing estuarine lake. Height differences between the coeval ridges at sites around the lake reflect local environmental controls including the range of wind fetch distances across shallow lake compartments, wave height and wind-induced seiche effects.

Lake size, macrophytes, and omnivory contribute to food web linkage in temperate shallow eutrophic lakes

Physical, chemical and biological processes facilitate cross-habitat connections in lakes, prompting food webs to be supported by different subsidies. We tested the hypothesis that the pelagic food web is subsidized by littoral resources and fish foraging behaviour plays a major role in carbon flux and on food web structure in shallow hypereutrophic lakes. We performed a fish diet and carbon and nitrogen isotope analyses to predict the linkage between littoral and pelagic habitats in three shallow hypereutrophic lakes. Lakes differed in morphology, fetch, macrophyte composition and width of the littoral zone. δ13C signals of seston differed among lakes, but were similar to other producers. Macroinvertebrates and fish carbon signatures were more enriched in the lake co-dominated by emergent and submerged vegetation. Fish foraging behaviour indicates that more than the 80% of the carbon that sustain adult fish was channelled from the littoral. In conclusion, littoral carbon were relevant and sustain, in part, food web in these shallow lakes. Factors like the extension of the littoral zone, lake morphometry, and the dominance of multi-chain omnivorous fish facilitate the connection among lake compartments and the transference of littoral carbon to lake food web.

Sampling sufficiency for estimating zooplankton diversity in neotropical floodplain lakes

AbstractThis study focuses on determining how many samples are needed to effectively assess the species richness of a community. Zooplankton samples at 40 sampling sites distributed among four lakes in the floodplain of the middle Araguaia River (Central‐West region of Brazil) were evaluated to determine the effect of the accumulation of collecting points by lake on species richness estimates. The results indicated the zooplankton community has high spatial heterogeneity. Thus, using a single sampling unit per lake would not be sufficient to accurately estimate their diversity (i.e. the zooplankton composition in these cases would be represented mainly by abundant species). Sampling designs that include a minimum of seven sampling sites in each lake are needed to record 70% of the total species richness. It is recommended, therefore, that researchers use a larger number of sampling sites per lake or, alternatively, that the water obtained and filtered through plankton nets is extracted from a wide area and in different lake compartments, rather than from a single site.

SimpleBox 4.0: Improving the model while keeping it simple…

Chemical behavior in the environment is often modeled with multimedia fate models. SimpleBox is one often-used multimedia fate model, firstly developed in 1986. Since then, two updated versions were published. Based on recent scientific developments and experience with SimpleBox 3.0, a new version of SimpleBox was developed and is made public here: SimpleBox 4.0. In this new model, eight major changes were implemented: removal of the local scale and vegetation compartments, addition of lake compartments and deep ocean compartments (including the thermohaline circulation), implementation of intermittent rain instead of drizzle and of depth dependent soil concentrations, adjustment of the partitioning behavior for organic acids and bases as well as of the value for enthalpy of vaporization. In this paper, the effects of the model changes in SimpleBox 4.0 on the predicted steady-state concentrations of chemical substances were explored for different substance groups (neutral organic substances, acids, bases, metals) in a standard emission scenario. In general, the largest differences between the predicted concentrations in the new and the old model are caused by the implementation of layered ocean compartments. Undesirable high model complexity caused by vegetation compartments and a local scale were removed to enlarge the simplicity and user friendliness of the model.

Lipid‐rich zooplankton subsidise the winter diet of benthivorous Arctic charr (Salvelinus alpinus) in a subarctic lake

SummaryGeneralist fish species commonly act as important links between littoral and pelagic habitats and food‐web compartments in lakes. However, diet and habitat links may depend significantly on seasonal availability of, and qualitative differences between, littoral and pelagic prey and on fish size. Despite increasing interest in food‐web dynamics, little is known about the seasonal changes in, or qualitative differences between, littoral and pelagic trophic pathways supporting generalist fish species in high‐latitude lakes.We used stomach contents together with analyses of stable carbon and nitrogen isotopes and fatty acids to study the winter and summer diet of generalistArctic charr and determine the qualitative differences between littoral and pelagic prey items. We were particularly interested to determine whetherArctic charr are able to utilise abundant and lipid‐rich winter zooplankton resources in subarcticLakeSaanajärvi, northernFinland.Arctic charr fed actively on cladoceran zooplankton in both seasons, despite the higher abundance and higher lipid content of calanoid copepods. Although the stomach contents consisted mainly of zooplankton in summer, the isotopic compositions of muscle and liver suggestArctic charr relied more on littoral carbon sources throughout the year. Fatty acid analysis indicated thatArctic charr had lower amounts of body fat and total and essential fatty acids in winter compared with summer.Observed seasonal feeding activity and dietary shifts were partly related toArctic charr size. Small (<200 mm)Arctic charr had more empty stomachs in winter, but higher amounts of zooplankton in stomachs and of essential fatty acids in muscle tissue in summer compared with larger (>200 mm) conspecifics that had more seasonally stable feeding activity and diet.Fatty acid analysis indicated that both littoral and pelagic food sources provided similar fatty acids toArctic charr, but in general, zooplankton had higher percentages of essential fatty acids compared with zoobenthos. PelagicEudiaptomus graciloidescalanoids and littoralGammarus lacustrisamphipods had the highest concentrations of total and essential fatty acids, but only the latter prey item was found in Arctic charr stomach contents.Our study demonstrates that lipid‐rich zooplankton can subsidise the predominantly benthivorous diet of top consumers (hereArctic charr) in subarctic lakes. The results also demonstrate that littoral and pelagic trophic pathways can be highly integrated in high‐latitude lakes, as a result of the flexible foraging behaviour of top consumers such as Arctic charr.

Niche segregation of coexisting Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) constrains food web coupling in subarctic lakes

Summary1. Generalist fish species are recognised as important couplers of benthic and pelagic food‐web compartments in lakes. However, interspecific niche segregation and individual specialisation may limit the potential for generalistic feeding behaviour.2. We studied summer habitat use, stomach contents and stable isotopic compositions of the generalist feeder Arctic charr coexisting with its common resource competitor brown trout in five subarctic lakes in northern Norway to reveal population‐level and individual‐level niche plasticity.3. Charr and trout showed partial niche segregation in all five lakes. Charr used all habitat types and a wide variety of invertebrate prey including zooplankton, whereas trout fed mainly on insects in the littoral zone. Hence, charr showed a higher potential to promote habitat and food‐web coupling compared to littoral‐dwelling trout.4. The level of niche segregation between charr and trout and between pelagic‐caught and littoral‐caught charr depended on the prevailing patterns of interspecific and intraspecific resource competition. The two fish species had partially overlapping trophic niches in one lake where charr numerically dominated the fish community, whereas the most segregated niches occurred in lakes where trout were more abundant.5. In general, pelagic‐caught charr had substantially narrower dietary and isotopic niches and relied less on littoral carbon sources compared to littoral‐caught conspecifics that included generalist as well as specialised benthivorous and planktivorous individuals. Despite the partially specialised planktivorous niche and thus reduced potential of pelagic‐dwelling charr to promote benthic–pelagic coupling, the isotopic compositions of both charr subpopulations suggested a significant reliance on both littoral and pelagic carbon sources in all five study lakes.6. Our study demonstrates that both interspecific niche segregation between and individual trophic specialisation within generalist fish species can constrain food‐web coupling and alter energy mobilisation to top consumers in subarctic lakes. Nevertheless, pelagic and littoral habitats and food‐web compartments may still be highly integrated due to the potentially plastic foraging behaviour of top consumers.

Mercury in an ultraoligotrophic North Patagonian Andean lake (Argentina): Concentration patterns in different components of the water column

The deep, ultraoligotrophic piedmont lakes of Northern Patagonia (Argentina) are located in pristine and barely impacted areas, along a wide latitudinal range. Several studies have reported moderate to high total mercury (Hg) concentrations and contrasting methylmercury (CH 3Hg +) production in different lake compartments. Sources of Hg for western Patagonian terrestrial and aquatic ecosystems are still not clear; while point sources can be ruled out and atmospheric deposition is a plausible source, along with contribution from active volcanic areas of the Andes. In this investigation, we reported a noticeably seasonal, spatial (between lake branches), and vertical (between water column strata) heterogeneity in the total Hg concentrations found in the pelagic zone of Lake Moreno. Sterile water samples taken in a depth profile of the lake showed moderate to high concentrations of CH 3Hg + in autumn with a decreasing trend with depth. Our results indicated that Hg is largely allocated in the plankton fraction between 10-53 μm; which dominated within the euphotic (epilimnetic) zone of the lake due to the high densities attained by two species of the dinoflagellate Gymnodinium. The 53-200 μm planktonic size fraction (comprising rotifers, ciliates and immature stages of crustaceans) and the > 200 μm fraction (calanoid copepods and cladocerans) were found to bear strikingly lower total Hg concentrations, suggesting that the magnification of Hg at the planktonic consumer level is negligible.

On the contribution of phytoplankton and benthic biofilms to the sediment record of marker pigments in high mountain lakes

This study compares the marker pigment composition in three different lake compartments: the water column; the surface sediment biofilm (0–0.5 cm), where production and main degradation processes take place; and the deep sediment (15–17 cm), where the signal is finally stored and marker pigments are used as a proxy in paleolimnological studies. The aim was to evaluate which marker pigments better record in the sediment the relative contributions of planktonic and benthic production in high mountain lakes, taking into account the differential preservation existing among pigments. A survey of 82 high mountain lakes distributed along the major environmental gradients was carried out in the Pyrenees. Comparison of pigments between the three compartments revealed that both water column and sediment biofilm signals could be distinguished in old sediment layers, despite the observation that some of the most characteristic carotenoids that appeared frequently and in high concentrations in the water column were already less common or even absent from the sediment biofilm. In the shallowest lakes, pigment composition in the sediment biofilm was typical of photosynthetically active communities and their deep sediment samples were characterised by a substitution of fucoxanthin by diatoxanthin as the dominant diatom marker pigment. However, in the deepest lakes the pigment composition of the sediment biofilm and deep sediment was similar, characterised by marker pigments of mainly planktonic algal groups and pheophytins, which are typical pigments of decaying communities. Results are discussed in terms of how pigment source (planktonic or benthic) and pigment type (lability of molecule) interact to shape pigment composition in the sediment record of high mountain lakes.

Lead biogeochemistry in a central Ontario Forested watershed

To determine the sources and sinks of atmospherically deposited Pb at a forested watershed (Plastic Lake) in central Ontario, Canada, Pb pools and fluxes through upland, wetland and lake compartments were measured during 2002/2003 and compared with previous measurements taken between 1989 and 1991. In 2002/2003, annual bulk deposition of Pb was 0.49 mg m−2 compared with 1.90–1.30 mg m−2 in 1989–1991. Annual Pb concentrations in stream water draining the upland part of the catchment were very low (0.04 μg l−1) and were approximately half those measured in 1989–1991 (0.11–0.08 μg l−1). Leaching losses in stream water were small and mass balance estimates indicate almost complete retention (>95%) of atmospherically deposited Pb in upland soils. In contrast, annual Pb concentrations in stream water draining a wetland were between 0.38 and 0.77 μg l−1, with the highest concentration occurring in 2002/2003 and mass balance calculations indicate that the wetland is a net source of Pb in all measured years. Lead concentrations in the lake outflow were low and the average Pb concentration measured in 2002/2003 (0.09 μg l−1) was approximately half the value recorded in 1989–1991 (0.19 μg l−1 both years). Annual mass balance estimates indicate that the lake retained between 2.47 mg m−2 (1989/1990) and 1.42 mg m−2 (2002/2003) and that in 2002/2003 68% of the Pb input to the lake is derived from the terrestrial catchment. These estimates are higher than sediment core records, which indicate around 18 mg m−2 Pb was retained in sediment during the 1990s. Nevertheless, Pb concentrations decrease with sediment depth and 206Pb/207Pb concentrations increase with depth, a pattern also observed in mineral soils that reflects the substantial contribution of anthropogenic Pb to the watershed. Lead isotope data from soil and sediment indicate a recent anthropogenic Pb signal (206Pb/207Pb ∼ 1.185) in upper soils and sediments and an older anthropogenic signal (206Pb/207Pb ∼ 1.20) in deeper soil and sediment. Lead isotope data in sediment and vegetation indicate that practically all the Pb cycled in the forest at Plastic Lake is anthropogenic in origin.

Heavy metals in Lake Balaton: water column, suspended matter, sediment and biota

During the period 1999–2002, five sampling cruises have been carried out on Lake Balaton to assess trace metal distribution in the lake and to identify major sources. Eighteen elements, including Cr, Co, Ni, Cu, Zn, Cd, Pb (trace metals) and Al, Ba, Ca, Fe, K, Mg, Mn, Na, P, S, Sr (major metals), were determined in one or more of the lake's compartments. Lower trace metal concentrations in rainwater were observed in June and February 2000, while much higher levels were present in September 2001 (during a storm event) and in snow (February 2000). In the Northern and Western parts of the lake, especially at the inflow of river Zala and the locations of the yacht harbours, metal concentrations were higher in almost all compartments. Because the lake is very shallow, storm conditions also change significantly the metal distributions in the dissolved and particulate phases. The Kis-Balaton protection system located on Zala river functions very efficiently for retaining suspended particulate matter (SPM; 72% retention) and associated metals. Metal concentrations in surface sediments of the lake showed a high variability. After normalisation for the fine sediment fraction, only a few stations including Zala mouth appeared to be enriched in trace metals. In zooplankton, Zn seemed to be much more elevated compared to the other trace metals. Based on the molar ratios of the trace metals in the various compartments and input flows of the lake, several trends could be deduced. For example, molar ratios of the trace metals in the dissolved and solid (suspended particulate matter and sediments) phases in the lake are fairly similar to those in Zala River.

Bacterial and protozoan mass accumulation in the deep chlorophyll maximum of a mesotrophic lake

Deep chlorophyll maxima (DCM) represent high accumulations of algal biomass in the microaerobic metalimnion of mesotrophic lakes characterised by a few algal species which persist in time. We studied the accumulation of bacteria, protozoans and the mesozooplankton associated with the DCM of mesotrophic Grosser Vatersee in northern Germany over a three year period and related our findings to the pattern found in the epilimnion and the hypolimnion. More than 20 % of the bacterial and protozoan biomass was concentrated in the narrow layer of the DCM, which comprised less than 10% of the whole water column. Between 16 and 36% of the rotifers and 12-31% of the juvenile cyclopoid copepods inhabited the DCM permanently on a day/night time scale, while calanoid copepods and adult cyclopoid copepods concentrated in the upper metalimnion and migrated into the DCM occasionally. Although bacteria and flagellates were generally larger in the DCM than in the epilimnion, ratios between bacterial and protozoan biomass were similar in both lake compartments. Our findings confirm a uniform community structure in the DCM of ciliates (Dexiotricha spp. and Strobilidium spp. comprised on average 70% of ciliate biomass) and, to a lesser extent, of mesozooplankton (rotifers and juvenile cyclopoid copepods dominated in the DCM). In contrast seasonal and interannual patterns of bacterial and protozoan biomass and cell size imply variation in the effects of predation.

Occurrence of high concentrations of a unique degradation product of chlorophyll-a in particles residing below the thermocline throughout a period of oxygen depletion in Lake Kinneret

An unrecognized chlorophyllous pigment, designated Pheob4, was the most abundant degradation product of chlorophyll-a (Chla) in the hypolimimnion, benthic boundary layer and sedimenting particles in Lake Kinneret in June-July 1997, when a prolonged period of residual oxygen concentration was found below the thermocline. The highest concentration of Pheob4 was found in the benthic boundary layer, where it reached as high concentration as 2 μg l-1. In sediment traps positioned below the thermocline Pheob4 constituted from about 5 to 20% of the Chl-a concentration and in bottom sediments from 5 to 185%. Pheob4 was not detected in epilimnetic water samples, but was found in sediment traps located within the oxygenated water column, within the range of surface wave action. Comparison of regular traps, with traps poisoned by formaldehyde indicates that the importance of Pheob4- containing particles in the sedimentation flux is relatively low, and that the bulk of Pheob4 found in the bottom sediment is apparently a result of Chl-a transformation in situ by biological activity. The timing and location of Pheob4 appearance coincided with the lake compartments where denitrification apparently took place. This suggests that Pheob4 may be used as a target pigment to trace organic matter processing under transient conditions in the water column and on the bottom sediments. Pheob4 was tentatively identified as 132, 172 cyclophaeophorbide-a enol. But, since the identification was done on a different system than the one used for the analysis of most of the samples, further investigation is required to verify that finding.

CYCLOPHEOPHORBIDE A IN SUSPENDED PARTICLES IN THE META‐ AND HYPOLIMNION DURING OXYGEN DEPLETION IN A SUBTROPICAL LAKE

Ostrovsky, I. & Yacobi, Y. Z.Israel Oceanographic and Limnological Research, Kinneret Limnological Laboratory, P.O. Box 345, Tiberias 1402, IsraelCyclopheophorbide a (CPP) was the most abundant degradation product of chlorophyll a (Chl a) in the hypolimimnion, benthic boundary layer and sedimenting particles in Lake Kinneret in June‐July 1997, when a prolonged period of residual oxygen concentration was found below the thermocline. The highest concentration of CPP was found in the benthic boundary layer, where it reached as high a concentration as 2 *g liter‐1. In sediment traps positioned below the thermocline CPP constituted from about 5 to 20% of the Chl a concentration and in bottom sediments from 5 to 185%. CPP was not detected in epilimnetic water samples, but was found in sediment traps located within the oxygenated water column, within the range of surface wave action. Comparison of regular traps, with traps poisoned by formaldehyde indicates that the importance of CPP‐containing particles in the sedimentation flux is relatively low, and that the bulk of CPP found in the bottom sediment is apparently a result of Chl a transformation in situ by biological activity. The timing and location of CPP appearance coincided with the lake compartments where denitrification apparently took place. This suggests that CPP can be used as a target pigment to trace organic matter processing under transient conditions in the water column and on the bottom sediments.

Physical determinants of the distribution and abundance of the burrowing ocypodid crab Paratylodiplax blephariskios Stebbing in the St. Lucia and Mhlathuze estuaries, KwaZulu-Natal, South Africa

The burrowing ocypodid crab, Paratylodiplax blephariskios, is endemic to the southeast coast of southern Africa where it forms an important component of the estuarine benthic fauna in muddy substrata. Two estuaries in KwaZulu-Natal, namely St. Lucia and the Mhlathuze, were chosen to investigate the influence of temperature, salinity, distance from the mouth and nature of the substratum (median phi value, sorting coefficient and organic content) on the distribution and abundance of the crab. Multiple and stepwise regression showed that particle size, sediment sorting coefficient and distance from the mouth were significantly related to crab abundance in both estuaries, while both temperature, salinity and the organic content of the substratum had little effect. At St. Lucia, distance from the mouth, rather than the nature of the substratum appeared to be the main determinant influencing crab distribution and abundance. However, at the Mhlathuze P. blephariskios was only recorded from muddy sites located further from the mouth. It was concluded that a combination of median particle size and sorting coefficient best explained crab distributions within the two estuaries. The absence of this crab species from muddy substrata in the lake compartments of the St. Lucia system suggests that its dispersion is tidal here, which leads to higher densities in the lower Narrows region near the mouth.