Differential responses of zooplankton populations (Bosmina longirostris) to fish predation and nutrient-loading in an introduced and a natural sockeye salmon nursery lake on Kodiak Island, Alaska, USA

Stratigraphic changes in the remains of Bosmina longirostris from a lake with an introduced sockeye salmon population and a lake with a natural salmon run on Kodiak Island demonstrated markedly different responses to past fluctuations in salmon populations. In both lakes, there was a positive correlation between the density of Bosmina microfossils and the abundance of sockeye salmon. However, opposite size trends were observed in the two lakes. In Karluk Lake, which has a native sockeye salmon population, Bosmina mean carapace lengths were largest at high salmon densities, and mean mucro and antennule lengths were also large, suggesting strong predation pressure from cyclopoid copepods, and less intense pressure from juvenile sockeye salmon. As salmon-derived nutrients are important in driving primary productivity in this system, changes in zooplankton productivity track salmon escapement, but grazing pressure on Bosmina from juvenile salmon is less important than that from cyclopoid copepods. In Frazer Lake, a lake with an introduced salmon population, Bosmina morphologies were smallest during periods of high sockeye salmon in the lake, suggesting much stronger predation effects from sockeye salmon due to the suppression of Cyclops columbianus. Latent development of compensatory mechanisms and the delayed recovery of copepod populations to salmon introductions has resulted in zooplankton populations that are still recovering from shifts in fish populations that occurred decades earlier. The differential response of Bosmina populations between the natural and manipulated lakes suggests that care must be taken when attempting to extrapolate results from whole-lake manipulations and short-term experiments to natural systems.     

<Emphasis Type="Italic">Bosmina</Emphasis> remains in lake sediment as indicators of zooplankton community composition

We measured Bosmina spp. mucro and antennule lengths in surface sediment samples from Wisconsin lakes to test whether such measures could be used to reconstruct zooplankton community composition and size structure in paleolimnological studies. Our data set included 58 lakes of various depths, water chemistry, trophic state, macrophyte cover, and zooplankton community composition. We used non-metric multidimensional scaling ordination (NMS) and simple correlation analysis to assess whether mucro and antennule measurements reflect the zooplankton community size structure. Bosmina mucro length (r = 0.727, p < 0.05) and antennule length (r = 0.360, p < 0.05) correlated with the NMS axis, which essentially represents zooplankton community size structure. Bosmina mucro length correlated positively with the abundance of the large-bodied zooplankter Epischura lacustris (r = 0.364, p < 0.01), as well as Diacyclops thomasi (r = 0.256, p < 0.05), and Leptodiaptomus minutus (r = 0.578, p ≤ 0.001), but correlated negatively with the abundance of the small-bodied zooplankter Tropocyclops prasinus (r = −0.385, p < 0.01). Bosmina antennule length correlated positively with the abundance of L. minutus (r = 0.344, p < 0.01) and negatively with T. prasinus (r = −0.258, p < 0.05). This broad, spatial scale assessment supports the use of Bosmina mucro and antennule lengths as a proxy for zooplankton community size structure. Mucro length is a stronger indicator of zooplankton community size structure as seen in its strong correlation with the NMS axis 1 and the significant correlations with abundance of predatory copepods.     

Cladoceran remains reveal presence of a keystone size-selective planktivore

We tested the use of cladoceran remains as a proxy for the presence and life history type of alewife (Alosa pseudoharengus) from pre-colonial times to present in a group of coastal lakes in southern New England. Alewife are a keystone predator that structure the zooplankton community through strong predation on large-bodied zooplankton species, which releases small zooplankton species, such as Bosmina spp., from competition and predation pressure. In southern New England there are lakes without alewife, lakes with anadromous alewife that only reside in lakes during the summer, and lakes with landlocked alewife that reside in lakes year-round. The entire zooplankton community of these lakes is structured differently based on the presence and type of alewife they contain. We examined differences in the morphology of Bosmina spp. from sediment core samples and contemporary zooplankton samples between lakes with different types of alewife. We found that there were significant differences in the morphology of Bosmina spp. between lakes with and without alewife. We also used discriminant analysis on the morphology of Bosmina spp. to classify lakes in terms of alewife presence and alewife type. We found that the morphology of Bosmina spp. can serve as a useful proxy for detecting the presence, but not the life history type of alewife from paleoecological and contemporary inferences.     

Fossil midge associations and the historical status of fish in acidified lakes

Chaoborid and chironomid (Diptera) fossils were examined in sediment cores form nine Ontario Precambrian Shield lakes that were fishless in 1979. An abundance of Chaoborus americamus (intolerant of predation by fish) throughout cores from four lakes indicated that they were fishless historically. Occurrence of Chaoborus punctipennis and C. flavicans and absence of C. americanus in cores, except near the surface in two cases, indicated that four other lakes were inhabited by fish historically. One of these was probably periodically fishless as suggested by fluctuations among these Chaoborus species. The ninth lake was not suitable for Chaoborus; only one specimen of C. trivittatus was found in this core. Chironomid fossil associations were substantially different in historically fishless and inhabited lakes. Fishless lakes had chironomids typical of eutrophic conditions indicating that seasonal oxygen depletion likely was inimical to fish. Chironomid associations of oligotrophic waters occurred in lakes historically inhabited by fish. Both good water quality and accessibility were required for long-term habitation by fish. Partial recovery of alkalinity and rise in pH were accompanied by substantial re-invasion by fish in two lakes which had been inhabited historically by fish. Diptera associations provide valuable information on the historical suitability of lakes for fish and the presence/absence of fish in regions where acidification of surface waters has occurred.     

Effects of fish introduction and eutrophication on the cladoceran community in Lake Fuxian,a deep oligotrophic lake in southwest China

Fish introduction and eutrophication are important disturbances to aquatic ecosystems, especially to oligotrophic plateau lakes that are generally considered to be very vulnerable ecosystems. Planktivorous fish Neosalanx taihuensis were introduced to Lake Fuxian, an oligotrophic (TP 17 μg/l) deep (average depth 89.7 m) plateau lake in southwest China, in the middle of the 1980s. After the introduction, N. taihuensis became the dominant fish species, and the total fish yield increased about threefold. Although the lake is still oligotrophic, the trophic state of Lake Fuxian has started to shift with increasing nutrient supply (eutrophication) due to an increase in human activities in the drainage basin. This study investigated the effects of N. taihuensis introduction and eutrophication on the cladoceran community of Lake Fuxian by examining changes in cladoceran assemblages and abundance, as well as the morphological features of Bosmina microfossils in the lake sediment. Absolute abundance of total Bosmina increased substantially after the middle of the 1980s. In addition, dominance of Bosmina with straight antennules was replaced by Bosmina with hooked antennules. The morphological variables (length of carapace, antennule and mucro) of Bosmina all decreased when planktivorous fish N. taihuensis achieved relatively large numbers. Eutrophication was the most important process determining cladoceran abundance, while fish introduction played an important role in structuring the cladoceran community in this oligotrophic, deep plateau lake.     

Proximity to ice fields and lake depth as modulators of paleoclimate records: a regional study from southwest Yukon,Canada

Pronounced climate warming during the past century has been well documented in high-latitude regions. Nonetheless, considerable heterogeneity exists in northern climate trends. We examined the roles of cryospheric landscape and lake depth in modulating the rate and magnitude of local climate responses through a paleolimnological study of lakes from southwest Yukon, Canada. By sampling lakes at varying distances from the Wrangell-St. Elias ice fields, we hypothesized that, for lakes of similar maximum depth, sites closest to the ice fields would be relatively complacent in terms of their chironomid and diatom assemblage changes over the past ~200 years. This hypothesis is based on the moderating effect of the glaciers on local climate, which would be most pronounced in the lakes nearest to the ice fields. However, given the known ecological differences between deep and shallow lakes, we further predicted that, for a given distance from the ice fields, a sediment record from a shallower lake would show the greatest change in stratigraphic subfossil assemblages. Because of the complicated shape of the ice fields, we applied the longitude for each site (which decreases from west to east) to approximate the proximity of our study lakes to the ice fields. Consistent with our predictions, we observed a space-transgressive pattern in the chironomid assemblage turnover that was associated with their proximity to the ice fields (r = ?0.75, P = 0.034, n = 8) across lakes of similar depth (mean maximum depth ± 1, SE = 18.1 ± 2.6 m). Considering a broader network of lakes that represented a greater range in maximum depth (4.9–29 m), we found that differences in subfossil chironomid assemblages between the modern and ca. AD 1800 sediment layers were strongly related to lake depth (r = ?0.77, P < 0.001, n = 15), but failed to detect a significant relationship with latitude or longitude (i.e. our proxy for proximity to the ice fields). Similarly, our comparative high-resolution analyses of two lakes with distinct lake morphometries, but similar proximities to the ice fields, demonstrated the predicted contrasting pattern: we observed pronounced post-1880 changes in the biotic assemblages in the shallow lake and a muted and delayed response (i.e. ~1970s) in the deeper lake. Our findings confirm that cryospheric landscape features can strongly modulate regional climate. Furthermore, our work shows that investigators need to be conscious of how climate change affects the structure and functioning of lakes of different typologies, which influences the way in which paleoclimate signals are recorded and interpreted.     

Cladoceran remains in lake sediments: a comparison between plankton counts and sediment records

To evaluate the comparability of neo- and paleo-limnology, we made year-to-year as well as seasonal comparisons of contemporary zooplankton data and cladoceran remains in thick (9–42 mm) annual laminations in sediment of Lake Vesijärvi, southern Finland. We calculated the expected annual exuviae production of nine planktonic taxa in the water column using contemporary zooplankton records, and compared the value to the observed net accumulation of their remains in deep sediment for 7 years. Although all of the species studied occur commonly in the lake pelagic zone, deposition of remains differed significantly among taxa. The observed accumulation of three Bosmina species and Chydorus sphaericus was similar to or exceeded expected values, suggesting good preservation of their remains as well as focusing of sediment into the deepest part of the lake. The accumulation of Limnosida and Leptodora remains exceeded expected values several fold, suggesting under-representation of these species in pelagic plankton samples, as well as efficient transportation and spatial averaging of their remains in the sediment. Daphnia, Diaphanosoma, and Ceriodaphnia were clearly under-represented in the sediment due to poor preservation of their remains and use of a 50-μm sieve in sediment processing. Thus, sieving should be taken into account as a possible source of bias. Correlation between relative abundances of all species in the sediment and in the plankton was weak (r _s = 0.54, P < 0.001), but for well-preserved species, the correlation was strong (r _s = 0.91, P < 0.001). Inter-annual variation in the deposition of remains suggested that resuspension and sediment focusing may vary between years, thus making it difficult to interpret absolute abundances, even though the deepest part of the basin shows clear varve formation and seasonality is well recorded. Detailed study of the uppermost lamination showed the seasonal succession of a cladoceran community within an annual varve as well as differences in the seasonality and intensity of ephippia production among the species. We recommend that results of cladoceran analyses be expressed in several ways (relative abundances, per unit dry weight, per unit organic matter, and as net accumulation values) before drawing final conclusions, as each approach may reveal a different aspect of the deposition process. In addition, sedimentation differences between epilimnetic and hypolimnetic species should be considered in stratified lakes.     

Subfossil Cladocera from surface sediment in thermokarst lakes in northeastern Siberia,Russia, in relation to limnological and climatic variables

Subfossil Cladocera were sampled and examined from the surface sediments of 35 thermokarst lakes along a temperature gradient crossing the tree line in the Anabar-river basin in northwestern Yakutia, northeastern Siberia. The lakes were distributed through three environmental zones: typical tundra, southern tundra and forest tundra. All lakes were situated within the continuous permafrost zone. Our investigation showed that the cladoceran communities in the lakes of the Anabar region are diverse and abundant, as reflected by taxonomic richness, and high diversity and evenness indices (H = 1.89 ± 0.51; I = 0.8 ± 0.18). CONISS cluster analysis indicated that the cladoceran communities in the three ecological zones (typical tundra, southern tundra and forest-tundra) differed in their taxonomic composition and structure. Differences in the cladoceran assemblages were related to limnological features and geographical position, vegetation type, climate and water chemistry. The constrained redundancy analysis indicated that T_July, water depth and both sulphate (SO₄ ^2?) and silica (Si⁴⁺) concentrations significantly (p ≤ 0.05) explained variance in the cladoceran assemblage. T_July featured the highest percentage (17.4 %) of explained variance in the distribution of subfossil Cladocera. One of the most significant changes in the structure of the cladoceran communities in the investigated transect was the replacement of closely related species along the latitudinal and vegetation gradient. The results demonstrate the potential for a regional cladoceran-based temperature model for the Arctic regions of Russia, and for and Yakutia in particular.     

Diatom-inference models for acid neutralizing capacity and nitrate based on 41 calibration lakes in the Sierra Nevada, California, USA

We investigated relationships among modern diatom species composition and physical and chemical characteristics of high-elevation lakes of the Sierra Nevada (California), to develop transfer functions that can be used to infer historic lake conditions. Data were collected from 50 lakes in National Parks and Forests of the central and southern Sierra Nevada. Multivariate statistical methods revealed that acid neutralizing capacity (ANC) and nitrate accounted for significant variation in diatom taxa. A training set with 242 modern diatom taxa from a subset of 41 lakes was used to develop transfer functions for ANC and nitrate using weighted averaging techniques. ANC and nitrate calibration ranges were 23.0–137 μEq/L and 0.18–9.5 μM, respectively. Coefficients of determination for the models were: ANC: R² = 0.76, and R _jackknife ²  = 0.44; NO₃: R² = 0.67, and R _jackknife ²  = 0.27. The ANC model was applied to the top 50 cm of sediments in Moat Lake to reconstruct ANC from ca. AD 350 to 2005. The reconstruction suggests that ANC declined by about 40 % (101–60 μEq/L) between the 1920s and the 1960s and then recovered to pre-1920s levels during 1980–2000. The magnitude of this ANC excursion was the largest observed during the past 1,600 years. We hypothesize that temporal variations in ANC were influenced by: (1) changes in rates of acid deposition, especially nitric acid and (2) variations in the timing and magnitude of snowmelt runoff.     

Sedimentary cladoceran assemblages and their functional attributes record late Holocene climate variability in southern Finland

We examined long-term relationships between Cladocera and limnogeological and climate variables in a late Holocene lake sediment sequence to assess cladoceran responses to climate change and reconstruct past climate variations in southern Finland, near the Baltic Sea coast. Elemental composition, organic matter, and inferred water quality variables were used to constrain paleoenvironmental (until 4500 cal BP) conditions and human impact. Fossil Cladocera assemblages, ephippia, and body size trends were utilized to estimate mean July air temperature (T _Jul) variability, open-water season length, and water-temperature regimes. Results revealed stable cladoceran communities, dominated by Eubosmina, until 500 cal BP, followed by major assemblage changes including increases in Chydorus cf. sphaericus, Alonella spp., and Bosmina longirostris. Pb and Cu concentrations were associated temporally with increasing human impact and warming climate during the past few centuries, and were the most significant factors in explaining cladoceran community changes, based on redundancy analysis. A Cladocera-based T _Jul reconstruction estimated elevated temperatures for the end of the Holocene Thermal Maximum, a cooler period during ~3000–2000 cal BP, slightly increased temperatures during 1200–800 cal BP corresponding to the Medieval Climate Anomaly (MCA), and lower temperatures during the Little Ice Age (LIA), 800–200 cal BP. The reconstruction also suggests that significant climate warming took place during the twentieth century. Cladocerans mostly associated with warm periods included B. longirostris and Pleuroxus uncinatus, whereas Alona affinis, A. quadrangularis, and C. cf. sphaericus were associated with colder climate. Compared to a reference Chironomidae-inferred T _Jul reconstruction from the same region, discrepancies were apparent in amplitude of temperature change, as the temperature variability in the cladoceran-based record was muted until ~200 cal BP. During the LIA, increased ephippia and body size agreed with the temperature reconstruction, but suggested a severely shorter open-water season and reduced water temperatures compared to the preceding late Holocene episodes, even though the cladoceran T _Jul showed quite similar temperatures for these periods. Our results suggest that fossil cladoceran assemblages, ephippia, and morphological attributes respond sensitively to long-term climate fluctuations and this record reflects well the major climate events of the late Holocene and provides realistic paleoclimatic estimates for maritime southern Finland.     

Defining drivers of nitrogen stable isotopes (δ15N) of surface sediments in temperate lakes

The nitrogen stable isotopic signature (δ¹⁵N) of sediment is a powerful tool to understand eutrophication history, but its interpretation remains a challenge. In a large-scale comparative approach, we identified the most important drivers influencing surface sediments δ¹⁵N of 65 lakes from two regions of Canada using proxies that reflect watershed nitrogen (N) sources, internal lake microbial cycling and productivity. Across regions, we found that water column total nitrogen (TN),  %N in the sediments and lake morphometric variables were the best predictors of sedimentary δ¹⁵N, explaining 66 % of its variation. Significant relationships were also found between sediment δ¹⁵N and human-derived N load ( \( R_{{{\text{adj}} .}}^{2} \)  = 0.23, p < 0.001), the latter being a strong predictor of TN ( \( R_{{{\text{adj}} .}}^{2} \)  = 0.68, p < 0.001). Despite a relatively strong overall relationship, variation partitioning revealed an interesting difference in the dominant variable that influenced regional δ¹⁵N. Alberta lake sedimentary δ¹⁵N signature was dominated by human derived N load. In contrast, internal processing appeared to be more important in Quebec lakes, where sediment δ¹⁵N was best explained by  %N in the sediments and lake volume. Overall, our findings support the use of δ¹⁵N in paleolimnological investigations to reconstruct changing N sources to lakes but also highlight that regions may have distinctive drivers. Interpretations of sediment δ¹⁵N are likely to be strongest when multiple lines of evidence are employed and when placed in a regional context.     

Phantom midge-based models for inferring past fish abundances

We sampled living and subfossil phantom midge (Diptera: Chaoboridae) larvae from surface sediments of 21 small lakes in Southern Sweden to examine the influence of fish and selected abiotic variables on the abundance and species composition of chaoborid assemblages. We expected total Chaoborus abundance to be inversely correlated with fish abundance and Chaoborus species most sensitive to fish predation to be found only in fishless lakes. We aimed to use the observed relationships to develop models to reconstruct past fish abundances from chaoborid remains and the abiotic environment. C. flavicans occurred in almost every lake, whereas subfossil C. obscuripes were found in the surface sediments of only one fishless lake. The density of living C. flavicans larvae correlated negatively with fish abundance, lake order and size. The concentration of C. flavicans subfossils was negatively associated with pH, lake size, water transparency and fish abundance. Regression models that included lake morphometry and landscape position as additional predictors of fish abundance performed better than models that used only Chaoborus predictors. The explained variance in fish abundance varied from 52 to 86%. Leave-one-out cross-validation indicated moderate performance of the two best models. These models explained 51 and 56% of the observed untransformed fish density and biomass, respectively. In addition, all Chaoborus models were unbiased in closely following the 1:1 reference line in plots of observed versus predicted values. These results are a promising step in developing midge-based paleolimnological reconstructions of past fish abundance, and the approach might be improved by including chironomid remains in the models.     

Influence of industrial activity and pollution on the paleoclimate reconstruction from a eutrophic lake in lowland England,UK

Reliable estimates of Holocene temperatures are important for understanding past climate dynamics, the response of biota to climate change, and validating climate models. Chironomids in lake sediment cores are used widely to quantify past summer temperatures, for which high-latitude and/or high-altitude lakes, remote from human influence, are usually considered appropriate. Temperature inferences from lowland lakes are likely influenced by other variables, specifically eutrophication and industrial pollution, but their reliability has never been tested. We used a Norwegian chironomid-based transfer function (r ² = 0.91; RMSEP = 1.01 °C) to infer mean July air temperature over the last 200 years, using chironomid assemblages in a core collected from a polluted, nutrient-enriched lake at Speke Hall, Liverpool, England. The chironomid-inferred temperatures correlate significantly with the local instrumental temperature record and follow long-term national temperature trends. These results show that chironomids can be used to produce reliable estimates of past mean July air temperature, even when other variables have also influenced the composition of the chironomid community. These findings underline the value of chironomids as sensitive and reliable quantitative proxies for summer temperature.     

Paleolimnological evidence of the consequences of recent increased dissolved organic carbon (DOC) in lakes of the northeastern USA

As a result of reductions in sulfate deposition and changing climate, dissolved organic carbon (DOC) concentrations have increased in many lakes situated in forests of northeastern North America and northern Europe since the 1990s. Although this increase is well documented, the associated ecological implications remain unclear. In particular, DOC strongly influences the vertical temperature structure of lakes, with increasing DOC often leading to a shallower epilimnion. We investigated the effect of increased DOC concentrations on lake thermal structure using fossil diatom records from six remote Maine lakes. Sedimentary diatom profiles from three pairs of small (<0.5 km²) lakes were compared, with each pair containing one lake with a documented significant increase in DOC and the other experiencing no change in DOC since the early 1990s. Lake thermal structure was inferred from changes in the relative abundance of Discostella stelligera and Aulacoseira species, two diatom taxa that are associated with changes in thermal stratification. The three lakes without increasing DOC showed no change in diatom-inferred mixing depth over the past few decades. Of the lakes with documented increases in DOC, two showed the highest turnover in diatom community structure over time. Profiles from both of these lakes also indicated directional change in diatom-inferred mixing depth over the past 20 years, but the direction of change differed. This study demonstrates that recent increases in DOC have the potential to alter the physical and biological structure of lakes, but that these responses may differ across lakes.     

Seasonal climate inferences from high-resolution modern diatom data along a climate gradient: a case study

This study represents a step towards developing seasonal climate inferences by using high-resolution modern data sets. The importance of seasonal climate changes is highlighted by the instrumental record of a meteorological station close to our study site (lac du Sommet in the Laurentian Mountains, Québec, Canada): Between 1966 and 2001, May temperatures increased significantly by 3.1°C (r = 0.41, n = 35, p < 0.01) but annual mean temperatures only by 0.6°C (r = 0.21, n = 35, p > 0.05). Comparison of this instrumental record with fossil diatom assemblages in a sediment core from lac du Sommet showed that axis one of a principal component analysis (PCA) of the fossil diatoms was best correlated with wind velocity in June (r = 0.62, n = 19, p < 0.005) and that past diatom production was significantly enhanced in periods with colder July temperatures (r =  ?0.77, n = 19, p < 0.0005) and higher wind velocity in June (r = 77, n = 19, p < 0.0005). The strong impact of the spring and summer conditions on overall diatom composition and productivity suggests that seasonal lake responses to climate are more important than annual mean temperatures. However, the seasonal dynamics of diatom communities are not well understood, and seasonality is rarely inferred effectively from lake sediment studies. Our research presents a pilot study to answer a twofold question: Is it possible to identify diatom communities which are typical for warmer or colder seasonal climate using sediment traps, and if it is, can this knowledge be used to infer seasonal climate conditions from fossil diatom assemblages? To address these questions, the seasonal dynamics of diatom communities and water chemistry were studied using sediment traps and water samples at biweekly intervals in four lakes distributed along an altitudinal gradient in the Laurentian Mountains from May through October 2002. Date of ice break-up was significantly related to the diatom assemblages taken in spring and uncorrelated to other significant environmental variables. Summer water temperature, circulation of the water column and pH explained a significant part of the biological variance in summer, and total nitrogen (TN) explained most of the biological variance in autumn. To infer these variables, weighted averaging partial least squares models were applied to the seasonal data sets. Inferred ice break-up dates were significantly correlated with number of days below 0°C in April (r = 0.52, n = 19, p < 0.025), inferred circulation of the water column was significantly related to measured wind velocity in June (r = 0.64, n = 19, p < 0.005), inferred summer water temperature and inferred pH was significantly related to measured July air temperature (r = 0.50, r =  ?53, n = 19, p < 0.025) and inferred TN autumn concentrations had an inverse relationship to August temperatures (r =  ?0.53, n = 19, p < 0.01). This comparison of the historical record with diatom-inferred seasonal climate signals, based on the comparison of fossil diatom assemblages with modern sediment trap data of high temporal resolution, provides a promising new approach for the reconstruction of seasonal climate aspects in paleolimnological studies.     

An annual fossil record of production,planktivory and piscivory during whole-lake manipulations

Annual and subannual paleolimnological records of pigments and zooplankton were used to analyze three whole-lake manipulations. The relative abundance of cladoceran remains in recent seasonal laminae was significantly correlated with the relative abundance of species in the plankton (r ²=0.59,P<0.001). Comparison of food-web structure after piscivore introduction showed that there are multiple outcomes of predation and that final food-web structure depended on the strength of interaction between piscivorous and planktivorous fish. Intense predation on cyprinids (Phoxinus eos, P. neogaeus, Umbra limi) by largemouth bass (Micropterus salmoides) allowed large herbivores (Daphnia pulex) and invertebrate predators (Chaoborus punctipennis) to dominate. Analysis of fossil invertebrate morphology suggested that small grazers (Bosmina longirostris, Diaphanosoma birgei) were eliminated by invertebrate predators. Under moderate predation by rainbow trout (Oncorhynchus mykiss), cyprinids remained, only intermediate-size herbivores (D. rosea) increased in abundance, andBosmina persisted. In contrast to food-web manipulations, increased algal abundance resulting from watershed disturbance (road construction) did not alter the species composition or size-structure of fossil Cladocera.     

Chaoborus (Diptera: Chaoboridae) mandibles—paleolimnological indicators of the historical status of fish populations in acid-sensitive lakes

Paleolimnological analyses of Chaoborus mandibles were used to assess the status of fish populations over the previous 150 to 300 years in five lakes from the Adirondack region of New York State. Windfall Pond (pH 6.5) has not acidified and currently has viable fish populations. Big Moose Lake (pH 5.0) has acidified in recent years, and the number of fish populations declined from 14 in the 1930's to seven in the early 1980's. The occurrence of only migratory Chaoborus (subgenus Sayomyia) in the cores indicated the long term presence of fish populations in Windfall Pond and Big Moose Lake. Brooktrout Lake (pH 5.0), Deep Lake (pH 4.7) and Upper Wallface Pond (pH 4.8) have all acidified in recent years, and all three are currently fishless. Chaoborus (Sayomyia) was present throughout the Brooktrout Lake core, but the entirely limnetic species, C. americanus, appeared in the topmost interval in the core. The appearance of C. americanus in the top of the core indicated a recent elimination of fish from Brooktrout Lake, probably during the 1970's. Elimination of fish by the 1940's was inferred for Deep Lake because C. americanus appeared above the 1930 level and replaced C. trivittatus as the dominant. Dominance of C. americanus throughout the Upper Wallface Pond core indicated that planktivorous fish were never present. These results strongly suggest that stratigraphic analyses of Chaoborus mandibles provide a useful assessment of the general status of historical fish populations in Adirondack lakes. The technique should be useful in other regions, as well as for applications other than those concerning lake acidification.This is the eleventh of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D.F. Charles and D.R. Whitehead are guest editors for this series.     

Effects of spring warming and mixing duration on diatom deposition in deep Tiefer See,NE Germany

Monitoring during three meteorologically different spring seasons in 2012, 2013, and 2014 revealed that temperature increase in spring, which influences spring lake mixing duration, markedly affected nutrient availability and diatom deposition in a sediment trap close to the bottom of deep Tiefer See, NE Germany. Deposition of Stephanodiscus taxa and small Cyclotella taxa was much higher after late ice out and a deep, short lake mixing period in spring 2013, compared to that after gradual warming and lengthy lake mixing periods in spring 2012 and 2014, when only brief or marginal ice cover occurred. Availability of dissolved Si and P was 33 and 20 % higher, respectively, in 2013 compared to 2014. The observed relation between high (low) diatom deposition and short (lengthy) mixing duration in spring was applied to varved sediments deposited between AD 1924 and 2008. Low detrital Si content in trapped material and a sediment core enabled use of µXRF-counts of Si as a proxy for diatom silica. The spring mixing duration for 1951–2008 was derived from FLake-model calculations. The spring warming duration related to lake mixing was approximated from air temperatures for 1924–2008 using the dates when daily mean air temperature exceeded 5 °C (start) and 10 °C (end). Diatom silica deposition showed a significant (p < 0.0001) inverse linear relationship with the modeled spring mixing duration (R² = 0.36) and the spring warming duration (R² = 0.28). In both cases, the relationship is strengthened when data from the period of low diatom production (1987–2005) is excluded (R² = 0.59 and R² = 0.35). Part of this low diatom production is related to external nutrient supply that favored growth of cyanobacteria at the expense of diatoms. This approach shows that diatom Si deposition was strongly influenced by the availability of light and nutrients, related to the duration of lake mixing and warming in spring, during most of the studied period. The remaining unexplained variability, however, indicates that additional factors influence Si deposition. Further tests in other deep, temperate lakes are necessary to verify if this relation is a common feature and consequently, if diatom Si can be used as a proxy for spring mixing duration in such lakes.     

Persistence of clear-water,shallow-lake ecosystems: the role of protected areas and stable aquatic food webs

The roles of both landscape alteration and in-lake processes need to be considered in conservation strategies for shallow lakes in the prairie regions of North America. Here we focus on shallow lakes in west-central Minnesota, USA, highlighting the long-term ecological history and response to known landscape changes of a clear-water, macrophyte-dominated, shallow lake. Contemporary limnological data suggest the aquatic ecosystem has been very stable and fishless for the last ~15 years. Sediment proxies for primary production and ecological change confirm that a stable ecosystem likely prevailed for the last ~200 years. However, sedimentary indicators of catchment erosion detail a distinct response to land-use change during the conversion of native grassland to agricultural land, and following establishment of a protected waterfowl production area (WPA) around the lake. Post-WPA, the rate of sediment accrual decreased dramatically within 5–10 years and sources of organic matter were similar to those of the pre-settlement period. The aquatic ecosystem has been able to withstand nutrient enrichment and allochthonous inputs because stable trophic interactions have likely been in place for more than 200 years. We conclude that lack of hydrologic connectivity and isolated, small catchments are important factors in the promotion of clear-water shallow lake ecosystems, mainly because they prevent colonization by fish and associated ecological consequences. This study highlights the importance of managing both the landscape and in-lake processes to maintain stable, clear-water, shallow lakes.     

Historic nutrient loading and recent species invasions caused shifts in water quality and zooplankton demography in two Finger Lakes (New York, USA)

We investigated the paleolimnology of Owasco and Seneca Lakes (New York, USA) and compiled water-quality monitoring data to describe environmental change during the past two centuries. Trophic shifts were detected in the oligotrophic to mesotrophic range and were likely driven by nutrient loading and species invasion. Based on box core reconstructions, primary production increased in both lakes during the last century, which is evidenced by the amount, type and isotopic composition of material preserved in the sediment. Organic matter accumulation and its stable carbon isotopic composition, as well as carbonate abundance, began to increase during the 1960s in Owasco Lake and the 1850s in Seneca Lake. Further, the abundance of phytoplanktivorous cladocera subfossils increased beginning in the 1910s in Seneca Lake and in the 1960s in Owasco Lake. The different timing and magnitude of the trophic shifts likely resulted from contrasts in lake residence time and species assemblages between the two lakes. The increases in primary and secondary production paralleled, and are interpreted to reflect, increased allochthonous nutrient loading. However, nutrient loading was not detected in the water-quality data, perhaps because of strong uptake of phosphates by phytoplankton or due to limited data collection. Rapid changes in cladoceran subfossils and water quality were also detected during the last decade. Concurrent with the establishment of non-native Cercopagis pengoi (fishhook waterflea), phytoplanktivorous cladocera remains declined, and the mucrone length of Bosmina increased, consistent with predictions of increased invertebrate predation. Additionally, the post-1990 decline in sediment carbonate, increased Secchi depth and decreased chlorophyll a concentrations followed the establishment of filter-feeding Dreissena spp. (zebra and quagga mussels). Collectively, paleolimnological data and water-quality monitoring provided a more complete and consistent record of shifts in the productivity of Owasco and Seneca Lakes, which were useful to understand environmental changes over different time scales. Physical, geochemical and biological changes were temporally consistent among three cores collected from different locations in each lake, but differed in magnitude for several variables (e.g., grain size and cladoceran subfossils), which could reflect near-shore to offshore gradients.