Susanne Lildal Amsinck

Zooplankton as indicators in lakes: a scientific-based plea for including zooplankton in the ecological quality assessment of lakes according to the European Water Framework Directive (WFD)

With the implementation of the EU Water Framework Directive (WFD), the member states have to classify the ecological status of surface waters following standardised procedures. It was a matter of some surprise to lake ecologists that zooplankton were not included as a biological quality element (BQE) despite their being considered to be an important and integrated component of the pelagic food web. To the best of our knowledge, the decision of omitting zooplankton is not wise, and it has resulted in the withdrawal of zooplankton from many so-far-solid monitoring programmes. Using examples from particularly Danish, Estonian, and the UK lakes, we show that zooplankton (sampled from the water and the sediment) have a strong indicator value, which cannot be covered by sampling fish and phytoplankton without a very comprehensive and costly effort. When selecting the right metrics, zooplankton are cost-efficient indicators of the trophic state and ecological quality of lakes. Moreover, they are important indicators of the success/failure of measures taken to bring the lakes to at least good ecological status. Therefore, we strongly recommend the EU to include zooplankton as a central BQE in the WFD assessments, and undertake similar regional calibration exercises to obtain relevant and robust metrics also for zooplankton as is being done at present in the cases of fish, phytoplankton, macrophytes and benthic invertebrates.

Sub-fossils of cladocerans in the surface sediment of 135 lakes as proxies for community structure of zooplankton, fish abundance and lake temperature

To elucidate the possibilities of using zooplankton remains in the surface sediment to describe present-days community structure and population dynamics of zooplankton, fish abundance and temperature, we compared contemporary data sampled in the pelagial during summer with the sediment record from the upper 1 cm of the sediment in 135 lakes covering a latitude gradient from Greenland in the north to New Zealand in the south. The abundance of three genera Bosmina, Daphnia and Ceriodaphnia of the total pool of ephippia was significantly related to the total abundance of the same taxa in the pelagic zone. However, in most lakes the abundance of Ceriodaphnia was higher in the sediment than in the water, which may be attributed to the overall preference by this genus for the littoral habitat. Using contemporary data from 27 Danish lakes sampled fortnightly during summer for 10 years, we found substantial inter-annual variations in the abundance of Daphnia spp., Ceriodaphnia spp., B. longirostris and B. coregoni. Yet, the sediment record mimicked the medium level well for most of the lakes, which suggests that the sediment record provides an integrated picture of the pelagic cladoceran community, which otherwise can be obtained only by long-term frequent contemporary sampling for several years. The contribution of Daphnia to the sum of Daphnia and Bosmina ephippia was negatively correlated with the abundance of fish expressed as catch per night in multi-mesh sized gill nets (CPUE). Yet, region-specific differences occurred, which partly could be eliminated by including nutrient state expressed as total phosphorus (TP) in a multiple regression. The average ratio of ephippia to the sum of ephippia and carapaces of Bosmina varied 40-fold between the sampling regions and was significantly negatively related to summer mean air temperature, and for Danish lakes also, albeit weakly, to fish CPUE but not to chlorophyll a. Apparently, temperature is the most important factor determining the ratio of parthenogenetic to ephippia producing specimens of Bosmina. We conclude that the sediment record of cladocerans is a useful indicator of community structure of pelagic cladocerans and the abundance of fish and temperature.

Reconstructing the historical changes in Daphnia mean size and planktivorous fish abundance in lakes from the size of Daphnia ephippia in the sediment

The zooplankton community structure in lakes is highly influenced by size-selective predation by fish, with small zooplankton species dominating at high predation pressure. Remains of cladocerans are preserved in the sediment and may be used to trace historical changes in fish predation. We determined how contemporary data on planktivorous fish were related to the size of Daphnia ephippia (dorsal length) in the surface sediment (0-1 cm) of 52 mainly shallow lakes with contrasting densities of fish and nutrients (TP: 0.002-0.60 mg P l-1). Density of fish expressed as catch per unit effort, in terms of numbers in multiple mesh-sized gill nets (CPUEn), decreased significantly with increasing mean size of ephippia. The relationship was improved by adding TP as an independent variable, now explaining 90% of the variation in CPUEn on the full data set covering lakes in Denmark, Greenland and New Zealand, and 78% if only data on Danish lakes were used. CPUE by weight of planktivorous fish and mean weight of Daphnia in the pelagial during summer were also related to ephippial size. By including contemporary data on established relationships between the sizes of egg-bearing female Daphnia and ephippia, we inferred changes in the CPUEn, mean size of ephippia-bearing Daphnia and summer mean body weight of Daphnia from ephippial size in four lakes during the past 1-2 centuries. In a hypertrophic lake subject to periodic fish kills, Daphnia mean body weight was high and CPUEn was low compared with those in two eutrophic lakes, while CPUEn was low and Daphnia body weight was high in the least eutrophic, clearwater lake. Estimated CPUEn and Daphnia mean weight in the surface sediment of these four lakes corresponded well with contemporary data. Only small changes in ephippial size with time were observed in the clearwater lake and in one of the lakes that had suffered early eutrophication, while major changes occurred in the two other lakes that had been subjected to a major increase in nutrient input or fish kills. We conclude that Daphnia ephippia preserved in the surface sediments of lakes may be a useful and efficient method to quantify the present-day abundance of planktivorous fish and Daphnia mean size. The method is particularly valid in surveys aimed to give a general picture of the fish stock and the ecological state in a set of lakes in a region rather than a precise estimate for a single lake. Though some evidence is provided, more work is needed to evaluate whether the equations are valid for hind-casting in down-core palaeoecological studies.

Fish and crustaceans in northeast Greenland lakes with special emphasis on interactions between Arctic charr (Salvelinus alpinus), Lepidurus arcticus and benthic chydorids

We studied the trophic structure in the pelagial and crustacean remains in the surface 1 cm of the sediment of 13 shallow, high arctic lakes in northeast Greenland (74° N). Seven lakes were fishless, while the remaining six hosted a dwarf form of Arctic charr (Salvelinus alpinus). In fishless lakes, Daphnia pulex was abundant, while no daphnids were found in the pelagial of lakes with fish. In fish lakes, the zooplankton community was dominated numerically by cyclopoid copepods and rotifers. Both lake sampling and analysis of remains in the top 1 cm of the sediment indicated that the phyllopod, Lepidurus arcticus, occurred in all fishless lakes, but was either absent or present in low densities from lakes with fish. Adult Lepidurus are mainly predators and forage in the top layer of the sediment. An analysis of surface sediment revealed low abundance of the benthic chydorids Alona sp. and Macrothrix sp. in lakes with Lepidurus, while they were abundant in lakes with fish. The low abundance in fishless lakes could not be explained by damage of crustacean remains caused by Lepidurus feeding in the sediment, because remains of the more soft-shelled, pelagic-living Daphnia were abundant in the sediment of these lakes. No significant differences between lakes with and without fish were found in chlorophyll a, total phosphorus, total nitrogen, conductivity or temperature, suggesting that the observed link between Lepidurus arcticus and the benthic crustacean community is causal. Consequently, remains of crustaceans in high arctic lake sediments may be useful for detecting the impact of past climate change on top-down control by fish. Not only remains of pelagic species, but also of Lepidurus and some benthic chydorids, may be used to detect changes in fish abundance and predation pressure in the past.

The role of cladocerans in tracking long-term change in shallow lake trophic status

Shallow lakes have been affected by a variety of human activities profoundly altering their ecological structure and function. Cladocerans have been used to track change resulting from a variety of drivers at a number of time scales. Aquatic macrophytes are well recognised as reflecting the ecological condition of a lake. Here, we compare the plant macrofossils with the sub-fossil cladoceran assemblages from 20 dated sediment cores. Co-correspondence analysis was used to determine the degree of commonality of change in community composition of the two biological groups through time. This analysis revealed very high levels of agreement in the nature and timing of change at all the sites examined with very high correlation coefficients between the axis 1 scores for macrofossils and cladocerans. Furthermore, at all sites a high proportion of the variance (min 20%, max 54%) in the macrofossil data was explained by the change in the cladoceran assemblage. Sub-fossil macrofossil and cladoceran assemblages, from at least from 1700 AD onwards, were examined in more detail at three sites: Ormesby Great Broad, Felbrigg Lake and Lake Søbygaard. There was very good accord in the main shifts of the cladoceran and macrofossil assemblages at all three sites. This may reflect the long-term shift in the principal focus of primary production from the benthic to the pelagic habitat. We suggest that the combination of their central position in the food-web and the presence of both pelagic and benthic taxa make cladocerans a strong candidate as the single best indicator of (palaeo) ecological condition related to changing trophic status and alteration in food-web structure in shallow lakes.

Lake Flora and Fauna in Relation to Ice-Melt, Water Temperature and Chemistry at Zackenberg

Publisher Summary The ecology of arctic lakes is strongly influenced by climate-generated variations in snow coverage and the duration of the ice-free period, which, in turn, affect the physical and chemical conditions of the lakes. The physical and chemical conditions in arctic lakes leave a very short growing season for aquatic organisms, and the role of climate is, therefore, very important. The nutrient concentrations are low in the arctic lakes because of the inflowing water that primarily originates from runoff of melting ice and snow. The melted water transports silt, inorganic and organic particles, as well as the atmospheric deposition of substances into the lakes. Most lakes have low average summer temperatures, low nutrient availability, and, with it, low primary production. This implies that species richness is limited and biomass is often relatively low compared to lakes in temperate regions. Therefore, interactions among organisms are less complex than elsewhere. Plants and animals are active before the ice melts. The increased influx of light in April–May means that sufficient light can penetrate the snow and ice layer, thereby initiating plant photosynthesis and a consequent phytoplankton biomass increase. This shows that light and not water temperature is the limiting factor for primary production prior to ice-out. Despite the low abundance of arctic charr, it has a strong regulating impact on the composition and density of the zooplankton community and the abundance of the arctic tadpole shrimp, Lepidurus arcticus , which likely regulates the density of benthic cladocerans.

Inference of past changes in zooplankton community structure and planktivorous fish abundance from sedimentary subfossils - a study of a coastal lake subjected to major fish kill incidents during the past century

To describe past changes in trophic structure, analyses were conducted of zooplankton (Cladocera, Rotifera) assemblages recovered in a 210 Pb dated sediment core from a shallow brackish lake exposed to several fish kill episodes during the 20 th century. A weighted-averaging zooplankton-based transfer function was applied to infer planktivorous fish densities (PL-CPUE) and to elucidate the potential of identify- ing fish kill incidents using sedimentary subfossils. The stratigraphical record showed a shift during the 20 th century. The cladoceran community structure changed from high abundance of macrophyte-associated taxa and chydorids, characteristic of clear water conditions, towards a community composed mainly by sediment-associated taxa, small-sized pelagic taxa as well as rotifers and chydorids, characteristic of eutrophic turbid conditions. Coinciding with these changes, the mean size of Daphnia ephippia decreased while inferred PL-CPUE increased. The results point towards progressive eutrophication with additional impact of salinity. Reduced temporal resolution of the sediment record made identification of fish kill incidents difficult. The indications of high fish planktivory and a decline in macrophyte abundance are consistent with anec- dotal observations of past fish investigations and contemporary data. The study dem- onstrates the usefulness of applying zooplankton subfossils to detect long-term changes in the trophic structure of brackish lakes in the past. In addition, it draws at- tention to the difficulties of using sediment records to highlight short-term events, such as fish kills, in shallow wind-impacted lakes.

Mid- to late-Holocene land-use change and lake development at Dallund Sø, Denmark: trophic structure inferred from cladoceran subfossils

Analyses of cladoceran remains were conducted on an 11-m sediment core from Dallund Sø Denmark, covering approximately the last 7000 years. The densities of planktivorous fish and macrophyte coverage were inferred from previously established transfer functions for Danish lakes using pelagic and plant-associated cladocerans, respectively, as palaeoenvironmental indicators. This is the first reconstruction of the abundance of fish and macrophytes covering millennial timescales. The cladoceran assemblages indicated an early period (4830 BC to c. 750 BC) with low species diversity, being dominated mainly by small-sized pelagic taxa. An intervening period (750 BC-AD 1100) followed, dominated by macrophyte-associated taxa and large-sized pelagic species. A marked increase in the abundance of remains occurred at C. AD 1200 coincident with the introduction of the mouldboard plough to Denmark and major forest clearance in the lake catchment. Further upcore (AD 1300-1700) mud-dwelling taxa increased in importance. Finally (AD 1700-1998), a shift occurred towards taxa characterizing eutrophic conditions. Redundancy analyses and cladoceran-inferred submerged macrophyte coverage and planktivorous fish density indicated overall low levels of nutrients and chlorophyll a, moderate macrophyte coverage (10-24%) and moderate to high fish predation prior to the Roman Iron Age (AD 0-400) followed by higher levels of nutrients and chlorophyll a and lower macrophyte coverage (< 10%) and moderate fish predation in recent times. The results suggest that the lake became increasingly eutrophic through time, not least after forest clearance and intensification of agriculture in Mediaeval times.

Cladoceran stratigraphy in two shallow brackish lakes with special reference to changes in salinity, macrophyte abundance and fish predation

Sub-fossils of Cladocera and Foraminifera were used to reconstruct changes since 1870 in the trophic dynamics of two brackish lakes, Glombak and Han Vejle, located in the Vejlerne nature reserve, Denmark, a site of international conservation importance. After creation of the lakes in the mid-1870s following land reclamation, the two lakes have developed quite differently; today Glombak is turbid, while Han Vejle is clear. In both lakes, stratigraphic changes in the assemblages of foraminifers and cladocerans indicate an abrupt shift from marine to brackish conditions at the end of the 19th century, coinciding with land reclamation. However, the composition of the fossil invertebrate assemblages in the 20th century implies differences in the exposure to salinity, in fish predation and in habitat diversity. In Glombak, the cladoceran record suggests relatively saline conditions in the first quarter of the last century and high macrophyte abundance followed by lower salinities and subsequently a major decrease in macrophyte abundance and an increase in fish predation during the past ca. 40 years. By contrast, in Han Vejle low salinity, high abundance of macrophytes and only minor changes in fish predation seem to have prevailed throughout most of the 20th century. The results are consistent with recent contemporary data, the few historical records, as well as with trends in the records of diatoms and macrofossils. This study highlights the potential of using crustacean remains as indicators of long-term changes in the trophic dynamics of brackish lakes.

Late Quaternary Environmental and Cultural Changes in the Wollaston Forland Region, Northeast Greenland

Publisher Summary This chapter reviews proxy data from a variety of natural archives sampled in the Wollaston Forland region, central Northeast Greenland. The data are used to describe long-term environmental and climatic changes, focusing on reconstructing the Holocene conditions, particularly in the Zackenberg area. The Wollaston Forland region is completely covered by the Greenland ice sheet several times during the quaternary. During the last glacial maximum (LGM), the margin of the inland ice might have extended to the shelf break off Northeast Greenland. Deglaciation of the Wollaston Forland commenced at the outer coast in the past 11,000–12,000 years, whereas the Zackenberg area was deglaciated somewhat later, in the past 10,000 years, and it took another 3,000–4,000 years before the inner fjords of the region were deglaciated. Isostatic rebound led to the rapid emergence in the early Holocene, but in the late Holocene, low-lying coastal areas were transgressed by the sea. The chapter discusses archaeological evidence for prehistoric occupation of the region.