Assessing lake eutrophication using chironomids: understanding the nature of community response in different lake types

1. Total phosphorus (TP) and chlorophyll a (Chl a) chironomid inference models ( Brodersen & Lindegaard, 1999 ; Brooks, Bennion & Birks, 2001 ) were used in an attempt to reconstruct changes in nutrients from three very different lake types. Both training sets were expanded, particularly at the low end of the nutrient gradient, using contemporary chironomid assemblages and environmental parameters from 12 British lakes, although this had little improvement on the model performances. 2. Dissimilarity analyses showed that the historic chironomid assemblages did not have good analogues in the original calibration or extended datasets. However, since the transfer functions are based on weighted averages of the trophic optima for the taxa present and not on community similarities, reasonable downcore inferences were produced. Ordination analyses also showed that the lakes retain their ‘identity’ over time, as the sample dissimilarities within lakes were less than the dissimilarities between lakes. 3. Analysis of the three historic lake profiles showed a range of chironomid community responses to lake development. Chironomids from a shallow lake, Slapton Ley, responded indirectly to nutrient enrichment (TP), probably through altered substrate, macrophyte and fish conditions, rather than directly to primary productivity (Chl a). A stratified lake, Old Mill Reservoir, showed a loss of the profundal chironomid fauna due to increasing primary productivity (Chl a) coupled with increasing hypoxia. A response to nutrients (TP or total nitrogen (TN)) at this site is also indirect, and the TP reconstruction therefore cannot be reliably interpreted. The third lake, March Ghyll Reservoir has little change in historic chironomid communities, suggesting that this well mixed, relatively unproductive lake has changed less than the other lakes. 4. Using chironomids to reconstruct nutrient histories does not follow a simple scheme. The response to changes in nutrients may be direct, but mediated through other ecosystem components. As alternative stable states are possible at a given level of TP it is also likely that alternative chironomid communities exist under similar nutrient conditions. Changes in biological communities can thus occur over thresholds, and it is only biological proxies that can reflect such ecosystem switches within palaeoenvironmental investigations.     

Submerged macrophytes as indicators of the ecological quality of lakes

1. We analysed submerged macrophyte communities from 300 Danish lakes to determine the efficacy of different species, maximum colonisation depth (C_max) of plants as well as coverage and plant volume inhabited (PVI) as indicators of eutrophication. 2. Most species occurred at a wide range of phosphorus and chlorophyll a (Chla) concentrations, but some species of isoetids (Lobelia, Isoëtes) and Potamogeton (Potamogeton gramineus, Potamogeton alpinus and Potamogeton filiformis) were mainly found at low nutrient concentrations and hence may be considered as indicators of nutrient poor conditions. However, species typically found in nutrient‐rich conditions, such as Elodea canadensis and Potamogeton pectinatus, were also found at total phosphorus (TP) <0.02 mg P L^?1 and Chla <5 μg L^?1 and therefore cannot be considered as reliable indicators of eutrophic conditions. 3. Submerged macrophyte coverage, PVI and the C_max were negatively correlated with TP and Chla. However, variability among lakes was high and no clear thresholds were observed. At TP between 0.03 and 0.07 mg P L^?1 plant coverage in shallow lakes ranged from nearly 0 to 100%, whilst at concentrations between 0.10 and 0.20 mg P L^?1 only 29% of the lakes had coverage >10%. C_max was found to be a useful indicator only in deep lakes with unvegetated areas in the deeper part, whereas the use of coverage was restricted to shallow lakes or shallow areas of deep lakes. 4. Overall, submerged macrophytes responded clearly to eutrophication, but the metrics investigated here showed no well‐defined thresholds. We developed a simple index based on species richness, presence of indicator species, coverage and C_max, which might be used to track major changes in macrophyte communities and for lake classification.     

Distribution and community structure of Ostracoda (Crustacea) in shallow waterbodies of southern Kenya

The current study presents the ostracod communities recovered from 26 shallow waterbodies in southern Kenya, combined with an ecological assessment of habitat characteristics. A total of 37 waterbodies were sampled in 2001 and 2003, ranging from small ephemeral pools to large permanent lakes along broad gradients in altitude (700–2 800 m) and salinity (37–67 200 µS cm^?1). Between 0 and 12 species were recorded per site. Lack of ostracods was associated with either hypersaline waters, or the presence of fish in fresh waters. Three of the 32 recovered ostracod taxa, Physocypria sp., Sarscypridopsis cf. elizabethae and Oncocypris mulleri, combined a wide distribution with frequent local dominance. Canonical correspondence analysis on species–environment relationships indicated that littoral vegetation, altitude, surface water temperature and pH best explain the variation in ostracod communities. Presence of fish and water depth also influence species occurrence, with the larger species being more common in shallow waterbodies lacking fish. Based on Chao’s estimator of total regional species richness, this survey recovered about two-thirds (60–68%) of the regional ostracod species pool. Scanning electron micrographs (SEM) of the valve morphology of 14 ostracod taxa are provided, in order to facilitate their application in biodiversity and water-quality assessments and in palaeoenvironmental reconstruction.     

Is the island biogeography model a poor predictor of biodiversity patterns in shallow lakes?

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A primary study on relationships between subfossil chironomids and the distribution of aquatic macrophytes in three lowland floodplain lakes,China

Aquatic macrophytes act as an important substrate for zoobenthos-supporting midge communities (chironomids) that can be used for reconstructing past environmental change. This study selected three macrophyte-dominated lakes in the middle reach of the Yangtze floodplain (central China), to identify relationships between macrophyte characteristics (i.e. communities, richness and biomass) and subfossil chironomid assemblages. One-way ANOSIM tests illustrated that there were significant differences in chironomid community compositions between lakes. Most chironomid taxa were not selective of dominant macrophyte species, but Corynoneura species were found to be associated with the development of Vallisneria as revealed by SIMPER analysis. Multivariate analyses indicate that submerged plant biomass, water depth and secchi depth, conductivity and the concentration of HCO₃ ^? were significantly correlated with midge compositions. Overall, this study corroborates the existing opinions that chironomid species show little selectivity to plant type but are significantly influenced by plant density. As an exception to this general trend, we found strong associations between Corynoneura chironomid larvae and Vallisneria macrophytes, prompting the need for future experimental studies to confirm this association.     

Inferring past zooplanktivorous fish and macrophyte density in a shallow lake: application of a new regression tree model

1. Eutrophication has a profound effect on the biological structure and function of shallow lakes, altering the composition and abundance of submerged macrophyte and fish assemblages. Relatively little is known, however, about decadal to centennial‐scale change in these important aspects of shallow lake ecology. 2. Established palaeolimnological inference models are limited to reconstructing a single variable. As macrophyte and zooplanktivorous fish abundance exert dual and interacting controls on cladoceran assemblages a single variable inference model may contain significant error. To obviate this problem, we applied a new cladoceran‐based multivariate regression tree (MRT) model to cladoceran subfossil assemblages from dated cores from a small shallow lake (Felbrigg Lake, U.K.) to assess long‐term change in fish and submerged macrophyte abundance. Plant macrofossil, chironomid and mollusc subfossil assemblages were also analysed to track changes in biological structure and function and to evaluate the inferences of the MRT model. 3. Over the 200+ year period covered by the sediment cores, there was good agreement in the timing and nature of ecological change reflected by the plant macrofossil, mollusc, chironomid and cladoceran data. The sediment sequence was divided into three dated zones: c. 1797–1890, c. 1890–1954 and c. 1954–present. Prior to 1890 plant‐associated mollusc, cladoceran and chironomid assemblages indicated a species‐rich macrophyte community; a scenario confirmed by the plant macrofossil data. From c. 1890 to 1954 macrophyte‐associated species of all three invertebrate groups remained abundant but the proportion of pelagic cladocerans rose. Post‐1954 mollusc and chironomid assemblages changed to sediment associated detrital feeders and the proportion of pelagic cladoceran taxa increased further. 4. The cladoceran‐based MRT model indicated a long period of stability, c. 1790–1927, characterised by abundant submerged macrophytes and zooplanktivorous fish. From c. 1927 to 1980, the MRT model inferred a decline in zooplanktivorous fish density (ZF) but relative stability in August macrophyte abundance. From 1980 to 2000, an increase in zooplanktivorous fish was inferred tallying well with available data on the fish population (since the 1970s), which indicated extirpation of perch in the 1970s and a subsequent increase in the rudd population. The model inferred little change in August macrophyte abundance until post‐c. 1980 at which point it indicated a decline. The surface sediment assemblage was placed in MRT group A, where submerged plants are absent or very rare in late summer in good agreement with current conditions at the site. 5. The MRT model, applied here for the first time, appears to have successfully tracked changes in macrophyte abundance and ZF over the last 200 years at Felbrigg Lake. The inferences agreed with historical observations on the fish community and the supporting palaeolimnological data. Given that multiple structuring forces shape most biological communities, the application of a model capable of allowing for this represents a significant advance in palaeolimnology.     

Bird abundance and species richness on Florida lakes: influence of trophic status,lake morphology,and aquatic macrophytes

Data from 46 Florida lakes were used to examine relationships between bird abundance (numbers and biomass) and species richness, and lake trophic status, lake morphology and aquatic macrophyte abundance. Average annual bird numbers ranged from 7 to 800 birds km^–2 and bird biomass ranged from 1 to 465 kg km^–2. Total species richness ranged from 1 to 30 species per lake. Annual average bird numbers and biomass were positively correlated to lake trophic status as assessed by total phosphorus (r = 0.61), total nitrogen (r = 0.60) and chlorophyll a (r = 0.56) concentrations. Species richness was positively correlated to lake area (r = 0.86) and trophic status (r = 0.64 for total phosphorus concentrations). The percentage of the total annual phosphorus load contributed to 14 Florida lakes by bird populations was low averaging 2.4%. Bird populations using Florida lakes, therefore, do not significantly impact the trophic status of the lakes under natural situations, but lake trophic status is a major factor influencing bird abundance and species richness on lakes. Bird abundance and species richness were not significantly correlated to other lake morphology or aquatic macrophyte parameters after the effects of lake area and trophic status were accounted for using stepwise multiple regression. The lack of significant relations between annual average bird abundance and species richness and macrophyte abundance seems to be related to changes in bird species composition. Bird abundance and species richness remain relatively stable as macrophyte abundance increases, but birds that use open-water habitats (e.g., double-crested cormorant, Phalacrocorax auritus) are replaced by species that use macrophyte communities (e.g., ring-necked duck, Aythya collaris).     

Trophic structure, species richness and biodiversity in Danish lakes: changes along a phosphorus gradient

1. Using data from 71, mainly shallow (an average mean depth of 3 m), Danish lakes with contrasting total phosphorus concentrations (summer mean 0.02–1.0 mg P L?l), we describe how species richness, biodiversity and trophic structure change along a total phosphorus (TP) gradient divided into five TP classes (class 1–5: <0.05, 0.05–0.1, 0.1–0.2, 0.2–0.4,> 0.4 mg P L?1).
2. With increasing TP, a significant decline was observed in the species richness of zooplankton and submerged macrophytes, while for fish, phytoplankton and floating‐leaved macrophytes, species richness was unimodally related to TP, all peaking at 0.1–0.4 mg P L?1. The Shannon–Wiener and the Hurlbert probability of inter‐specific encounter (PIE) diversity indices showed significant unimodal relationships to TP for zooplankton, phytoplankton and fish. Mean depth also contributed positively to the relationship for rotifers, phytoplankton and fish.
3. At low nutrient concentrations, piscivorous fish (particularly perch, Perca fluviatilis) were abundant and the biomass ratio of piscivores to plankti‐benthivorous cyprinids was high and the density of cyprinids low. Concurrently, the zooplankton was dominated by large‐bodied forms and the biomass ratio of zooplankton to phytoplankton and the calculated grazing pressure on phytoplankton were high. Phytoplankton biomass was low and submerged macrophyte abundance high.
4. With increasing TP, a major shift occurred in trophic structure. Catches of cyprinids in multiple mesh size gill nets increased 10‐fold from class 1 to class 5 and the weight ratio of piscivores to planktivores decreased from 0.6 in class 1 to 0.10–0.15 in classes 3–5. In addition, the mean body weight of dominant cyprinids (roach, Rutilus rutilus, and bream, Abramis brama) decreased two–threefold. Simultaneously, small cladocerans gradually became more important, and among copepods, a shift occurred from calanoid to cyclopoids. Mean body weight of cladocerans decreased from 5.1 μg in class 1 to 1.5 μg in class 5, and the biomass ratio of zooplankton to phytoplankton from 0.46 in class 1 to 0.08–0.15 in classes 3–5. Conversely, phytoplankton biomass and chlorophyll a increased 15‐fold from class 1 to 5 and submerged macrophytes disappeared from most lakes.
5. The suggestion that fish have a significant structuring role in eutrophic lakes is supported by data from three lakes in which major changes in the abundance of planktivorous fish occurred following fish kill or fish manipulation. In these lakes, studied for 8 years, a reduction in planktivores resulted in a major increase in cladoceran mean size and in the biomass ratio of zooplankton to phytoplankton, while chlorophyll a declined substantially. In comparison, no significant changes were observed in 33 ‘control’ lakes studied during the same period.     

Chironomid stratigraphy in the shallow and eutrophic Lake Søbygaard, Denmark: chironomid–macrophyte co-occurrence

1. A sediment core from the shallow, hypertrophic Lake Søbygaard (mean depth ∼1 m; [TP] 310 μg P L⁻¹) was analysed for subfossil remains to reconstruct chironomid community changes in relation to the succession and disappearance of aquatic macrophytes. 2. Species composition in the 1.10 m core indicates a succession from a 'naturally' eutrophic state to a hypertrophic state during recent centuries. Radiometric dating (²¹⁰Pb) of the uppermost 20 cm of the sediment core (∼1932–93) indicates that sediment accumulation rate had doubled in recent decades. 3. Changes in chironomid assemblages were in close agreement with changes in both diatoms and macrophyte remains in the same core. Distinct changes in chironomid communities reflect the eutrophication process and macrophyte succession through Chara , Ceratophyllum and Potamogeton dominance to the present state, with complete loss of submerged vegetation and dominance by phytoplankton. 4. The co-occurrence and relationship between aquatic macrophyte diversity and recent subfossil chironomid assemblages were assessed from an additional 25 Danish lakes. There was good agreement between the macrophyte and chironomid-based lake groupings. Overall, a significant difference ( P <0.001) was found in chironomid assemblages among lakes in different macrophyte classes. In a pair-wise comparison, the poorly buffered mesotrophic lakes and the alkaline eutrophic lakes had significantly different chironomid assemblages. 5. Chironomid taxa commonly reported to be associated with macrophytes ( Cricotopus , Endochironomus and Glyptotendipes ) were shown also to be indicators of highly productive lakes lacking abundant submerged vegetation.     

Effects of water-level regulation on the nearshore fish community in boreal lakes

The fish community in the littoral areas of eight regulated lakes and five reference lakes in Finland was sampled by electrofishing. No significant effect of winter drawdown on species richness was recorded across lakes. Total fish density for stony bottoms of the regulated and reference lakes averaged 19.3 and 32.7 individuals per 100 m², respectively, but this difference was not statistically significant. The combined proportion of littoral fish species, including minnow (Phoxinus phoxinus), bullhead (Cottus gobio), alpine bullhead (Cottus poecilopus), nine-spined stickleback (Pungitius pungitius), and stone loach (Barbatula barbatula), supplemented with zoobenthos feeders ruffe (Gymnocephalus cernuus) and young burbot (Lota lota), was much lower in the regulated lakes than in the reference lakes. Besides winter drawdown, other variables, such as nutrient level and lake size, affected the fish community. Guest editors: K. M. Wantzen, K.-O. Rothhaupt, M. M?rtl, M. Cantonati, L. G.-Tóth & P. Fischer Ecological Effects of Water-Level Fluctuations in Lakes     

Influence of nutrients, submerged macrophytes and zooplankton grazing on phytoplankton biomass and diversity along a latitudinal gradient in Europe

In order to evaluate latitudinal differences in the relationship of phytoplankton biomass and diversity with environmental conditions in shallow lakes, we sampled 98 shallow lakes from three European regions: Denmark (DK), Belgium/The Netherlands (BNL) and southern Spain (SP). Phytoplankton biomass increased with total phosphorus (TP) concentrations and decreased with submerged macrophyte cover across the three regions. Generic richness was significantly negatively related to submerged macrophyte cover and related environmental variables. Zooplankton:phytoplankton biomass ratios were positively related to submerged macrophyte cover and negatively to phytoplankton generic richness in DK and BNL, suggesting that the low generic richness in lakes with submerged macrophytes was due to a higher zooplankton grazing pressure in these regions. In SP, phytoplankton generic richness was not influenced by zooplankton grazing pressure but related to conductivity. We observed no relationship between phytoplankton generic richness and TP concentration in any of the three regions. The three regions differed significantly with respect to mean local and regional generic richness, with BNL being more diverse than the other two regions. Our observations suggest that phytoplankton diversity in European shallow lakes is influenced by submerged macrophyte cover indirectly by modulating zooplankton grazing. This influence of submerged macrophytes and zooplankton grazing on phytoplankton diversity decreases from north to south.     

Assessment of Littoral Benthic Invertebrate Communities at the Land–Water Interface in Lakes Recovering from Severe Acid- and Metal-Damage

Benthic invertebrate communities within confluence sites, or areas of sediment deposition, are shaped by the input of catchment products including coarse woody debris, organic and inorganic particulates, and contaminants, but these sites also appear to be potential “hotspots” where recolonization of severely damaged ecosystems begins. Two species of leaf packs and a sweep netting technique were used to assess benthic invertebrate communities across a gradient of 14 confluence sites in 3 recovering lakes near the copper and nickel smelters in Sudbury, Canada. Environmental variables including delta habitat composition, delta area and length, and composition of deposited materials were used to detect spatial patterns in littoral benthic invertebrate communities. Benthic invertebrate community relationships with water chemistry were also assessed. Partial redundancy analysis (pRDA) showed that all sampling methods detected similar gradients of increasing invertebrate community richness and diversity as area and length of the sediment delta and the surface organic matter abundance increased. Two-way nested ANOVAs showed significant differences (p < .05) in taxa richness and diversity metrics among sites. Of the three methods, the benthic invertebrate community measurements from the birch leaf packs provided the strongest correlations with measures of organic matter inputs or habitat characteristics of the confluence zones. These correlations suggest that tree planting in riparian areas, or organic matter or macrophyte additions to littoral zones, may enhance littoral benthic invertebrate richness and diversity in acid and metal damaged lakes.     

Subfossil chironomid assemblages in deep,stratified European lakes: relationships with temperature,trophic state and oxygen

1. The distributions of subfossil remains of chironomid larvae in 28 large, deep and stratified lakes in Europe were examined in surface sediments along a latitudinal transect ranging from northern Sweden to southern Italy. 2. Canonical correspondence analysis (CCA) showed that summer surface water and July air temperature, as well as total phosphorus (TP) concentrations, hypolimnetic oxygen availability and conductivity were statistically significant (P < 0.05) explanatory variables explaining between 11 and 14% of the variance in the chironomid data. 3. Owing to the spatial scale covered by our study, many environmental variables were covarying. Temperature, TP concentration and oxygen availability were positively or negatively correlated with the first axis of a detrended correspondence analysis (DCA) of chironomid assemblages, suggesting that climatic and trophic conditions influenced profundal chironomid assemblages either in a direct (food and oxygen) or in an indirect (temperature) way. Parameters related to local environmental conditions, lake morphology and bedrock geology, such as organic matter content of the sediment, maximum lake depth, Secchi depth and pH, were not significant in explaining the distribution of chironomid assemblages in our study lakes. 4. The strong relationship between chironomid assemblages and summer temperature may be related to the covariation of temperature with parameters, such as nutrient and oxygen availability, known to affect chironomid assemblages in deep, stratified lakes. However, summer temperature explained a statistically significant proportion of the variance in the chironomid assemblages even when effects of oxygen availability and TP concentrations were partialled out. This suggests that summer temperature has an effect on chironomid assemblages in deep lakes, which is not related to its covariation with trophic state. 5. The potential of fossil chironomid analysis for quantitatively reconstructing past nutrient conditions in deep, stratified lakes was examined by calculating the Benthic Quality Index (BQI) based on subfossil chironomids and by comparing BQI values with observed TP concentrations. BQI was linearly related to log‐transformed TP. Applying this relationship to fossil chironomid assemblages from Lake Päijänne (Finland) produced a TP reconstruction in agreement with measured TP during the period 1970–1990, demonstrating that this approach can provide quantitative estimates of past nutrient concentrations in deep, stratified lakes.     

Impacts of space,local environment and habitat connectivity on macrophyte communities in conservation lakes

Aim  To assess the relative impacts of spatial, local environmental and habitat connectivity on the structure of aquatic macrophyte communities in lakes designated for their conservation value. Location  Selected lakes of conservation importance all over Scotland, representing a wide variety of lake habitat types and associated macrophyte communities. Methods  Local environmental variables and species occurrence were measured in the field. Spatial variables were generated using principal coordinates of neighbour matrices (PCNM) analysis. Connectivity between each lake and its neighbours was defined as either (i) all lakes within a radius of 5, 10, 25, 50, 75 or 100 km; (ii) all lakes in same river system; or (iii) all lakes in the same catchment and upstream of the lake. Using variance partitioning within canonical correspondence analysis, the relative impact of E = local environment, S = space and C = lake connectivity was compared on submerged (n = 119 lakes) and emergent (n = 96 lakes) macrophyte assemblages. Results  Local environmental conditions, such as total phosphorus, alkalinity/conductivity and the presence of invasive species, as well as spatial gradients were key drivers of observed variation in macrophyte communities; e.g., for submerged macrophytes, a combination of local to moderate factors relating to water chemistry and broad‐scale gradients reflecting elevation and climate are important. Spatially structured environmental variables explained a large portion of observed variation. Main conclusions  Our findings confirmed the need to manage local environmental pressures such as eutrophication, but suggested that the traditional catchment approach was insufficient. The spatial aggregation of environmental and connectivity factors indicated that a landscape scale approach should be used in lake management to augment the risk assessment to conservation species from the deterioration of suitable lake sites over broad biogeographic areas.     

Responses of four submerged macrophytes to freshwater snail density (Radix swinhoei) under clear‐water conditions: A mesocosm study

Macrophytes play a key role in stabilizing clear‐water conditions in shallow freshwater ecosystems. Their populations are maintained by a balance between plant grazing and plant growth. As a freshwater snail commonly found in shallow lakes, Radix swinhoei can affect the growth of submerged macrophytes by removing epiphyton from the surface of aquatic plants and by grazing directly on macrophyte organs. Thus, we conducted a long‐term (11‐month) experiment to explore the effects of snail density on macrophytes with distinctive structures in an outdoor clear‐water mesocosm system (with relatively low total nitrogen (TN, 0.66 ± 0.27 mg/L) and total phosphorus (TP, 36 ± 20 μg/L) and a phytoplankton chlorophyll a (Chla) range of 14.8 ± 4.9 μg/L) based on two different snail densities (low and high) and four macrophyte species treatments (Myriophyllum spicatum, Potamogeton wrightii, P. crispus, and P. oxyphyllus). In the high‐density treatment, snail biomass and abundance (36.5 ± 16.5 g/m² and 169 ± 92 ind/m², respectively) were approximately twice that observed in the low‐density treatment, resulting in lower total and aboveground biomass and ramet number in the macrophytes. In addition, plant height and plant volume inhabited (PVI) showed species‐specific responses to snail densities, that is, the height of P. oxyphyllus and PVI of M. spicatum were both higher under low‐density treatment. Thus, compared with low‐density treatment, the inhibitory effects of long‐term high snail density on macrophytes by direct feeding may be greater than the positive effects resulting from epiphyton clearance when under clear‐water conditions with low epiphyton biomass. Thus, under clear‐water conditions, the growth and community composition of submerged macrophytes could be potentially modified by the manual addition of invertebrates (i.e., snails) to lakes if the inhibitory effects from predatory fish are minor.     

Subfossil Cladocera in relation to contemporary environmental variables in 54 Pan-European lakes

1. Changes in cladoceran subfossils in the surface sediments of 54 shallow lakes were studied along a European latitude gradient (36–68°N). Multivariate methods, such as regression trees and ordination, were applied to explore the relationships between cladoceran taxa distribution and contemporary environmental variables, with special focus on the impact of climate. 2. Multivariate regression tree analysis showed distinct differences in cladoceran community structure and lake characteristics along the latitude gradient, identifying three groups: (i) northern lakes characterised by low annual mean temperature, conductivity, nutrient concentrations and fish abundance, (ii) southern, macrophyte rich, warm water lakes with high conductivity and high fish abundance and (iii) Mid‐European lakes at intermediate latitudes with intermediate conductivities, trophic state and temperatures. 3. Large‐sized, pelagic species dominated a group of seven northern lakes with low conductivity, where acid‐tolerant species were also occasionally abundant. Small‐sized, benthic‐associated species dominated a group of five warm water lakes with high conductivity. Cladoceran communities generally showed low species‐specific preferences for habitat and environmental conditions in the Mid‐European group of lakes. Taxon richness was low in the southern‐most, high‐conductivity lakes as well as in the two northern‐most sub‐arctic lakes. 4. The proportion of cladoceran resting eggs relative to body shields was high in the northern lakes, and linearly (negatively) related to both temperature and Chl a, indicating that both cold climate (short growing season) and low food availability induce high ephippia production. 5. Latitude and, implicitly, temperature were strongly correlated with conductivity and nutrient concentrations, highlighting the difficulties of disentangling a direct climate signal from indirect effects of climate, such as changes in fish community structure and human‐related impacts, when a latitude gradient is used as a climate proxy. Future studies should focus on the interrelationships between latitude and gradients in nutrient concentration and conductivity.     

Distribution and faunal richness of Cladocera in western Uganda crater lakes

In this study, we analyse the distribution and species richness of epibenthic and planktonic Cladocera (Crustacea: Branchiopoda) in 62 Uganda crater lakes, spread across the climatic gradient between the sub-humid shoulder and semi-arid floor of the East African Rift Valley. Together, these lakes cover large environmental gradients in salinity, trophic conditions and depth. In total, 36 species of Cladocera were encountered in the freshwater lakes (<1,500 μS/cm), whereas only a single species was found in the true saline lakes (>10,000 μS/cm). Cladoceran species richness in individual lakes was found to be determined primarily by the presence of a well-developed littoral belt of submerged and emergent aquatic macrophytes, pH and salinity. The highest species richness occurred in fresh but eutrophic shallow waters, with relatively low pH (6.5–7) and dense aquatic macrophyte growth. As identified by multivariate statistical analysis, the distribution of Cladocera species among the Uganda lakes was most strongly determined by nutrient availability (measured as total phosphorus), the presence and diversity of aquatic macrophyte habitat, pH, mean annual temperature and the fraction of the crater catchment that is currently under agriculture. Since Cladocera play an important role in aquatic food webs, and as such contribute to the ecological integrity of aquatic ecosystems, an increased understanding of the environmental controls underlying their distribution provides valuable information on aquatic ecosystem functioning needed for management and conservation. The significant turnover of cladoceran species composition along the sampled environmental gradients demonstrates their potential as biological indicators for water quality and ecosystem health in East African lakes. Our results suggest that changes in land use are the greatest threat to natural ecosystem functioning in these African lakes, and particularly so in the shallower lakes.     

Seasonal dynamics of macrophytes and phytoplankton in shallow lakes: a eutrophication‐driven pathway from plants to plankton?

1. Seasonal relationships between macrophyte and phytoplankton populations may alter considerably as lakes undergo eutrophication. Understanding of these changes may be key to the interpretation of ecological processes operating over longer (decadal‐centennial) timescales. 2. We explore the seasonal dynamics of macrophytes (measured twice in June and August) and phytoplankton (measured monthly May–September) populations in 39 shallow lakes (29 in the U.K. and 10 in Denmark) covering broad gradients for nutrients and plant abundance. 3. Three site groups were identified based on macrophyte seasonality; 16 lakes where macrophyte abundance was perennially low and the water generally turbid (‘turbid lakes’); 7 where macrophyte abundance was high in June but low in August (‘crashing’ lakes); and 12 where macrophyte abundance was high in both June and August (‘stable’ lakes). The seasonal behaviour of the crashing and turbid lakes was extremely similar with a consistent increase in nutrient concentrations and chlorophyll‐a over May–September. By contrast in the stable lakes, seasonal changes were dampened with chlorophyll‐a consistently low (<10–15 μg L^?1) over the entire summer. The crashing lakes were dominated by one or a combination of Potamogeton pusillus, Potamogeton pectinatus and Zannichellia palustris, whereas Ceratophyllum demersum and Chara spp. were more abundant in the stable lakes. 4. A long‐term loss of macrophyte species diversity has occurred in many shallow lakes affected by eutrophication. One common pathway is from a species‐rich plant community with charophytes to a species‐poor community dominated by P. pusillus, P. pectinatus and Z. palustris. Such compositional changes may often be accompanied by a substantial reduction in the seasonal duration of plant dominance and a greater tendency for incursions by phytoplankton. We hypothesise a slow‐enacting (10–100 s years) feedback loop in nutrient‐enriched shallow lakes whereby increases in algal abundance are associated with losses of macrophyte species and hence different plant seasonal strategies. In turn such changes may favour increased phytoplankton production thus placing further pressure on remaining macrophytes. This study blurs the distinction between so‐called turbid phytoplankton‐dominated and clear plant‐dominated shallow lakes and suggests that plant loss from them may be a gradual process.     

The response of periphyton and submerged macrophytes to nitrogen and phosphorus loading in shallow warm lakes: a mesocosm experiment

1. Recent experimental and field studies on temperate shallow lakes indicate that nitrogen may play a greater role in their functioning than previously thought. Several studies document that abundance and richness of submerged macrophytes, both central in shallow lake ecology, may decrease with increasing nitrogen loading, especially at high phosphorus levels. However, the role of nitrogen in warm lakes with fluctuating water regimes remains to be described in detail. 2. The effect of increasing nitrate and phosphate concentrations on submerged macrophyte growth was examined in a 3‐month mesocosm experiment conducted in summer in a shallow freshwater lake on the north western coast of Turkey with a Mediterranean climate. Twenty four field mesocosms, open to the sediment and atmosphere, were stocked with Myriophyllum spicatum shoots and small cyprinid fish. Three nitrate loadings in combination with two phosphate loadings were applied in a fourfold replicated design. 3. Mean ± SD nutrient concentrations maintained throughout the experiment were 0.55 ± 0.17, 2.2 ± 0.97, 9.2 ± 5.45 mg L^?1 total nitrogen and 55 ± 19.2, 73 ± 22.9 μg L^?1 total phosphorus. Mean periphyton biomass increased with increasing nutrient concentrations and peaked at the highest nitrogen and phosphorus loadings, while the mean phytoplankton biomass remained relatively low in all treatments. 4. Percent volume inhabited (% PVI) by macrophytes throughout the experiment and total macrophyte biomass at the end of the experiment did not differ among treatments. In addition to stocked M. spicatum, Ceratophyllum demersum and Potamogeton crispus appeared in the majority of the mesocosms. The plants grew continuously up to 50% PVI throughout the experiment and remained resilient to shading provided by periphyton and phytoplankton. 5. The mean summer air temperature in 2007 was 2.2 °C higher than the average of the last 32 years, which resulted in a water level decrease of 0.3 m in the mesocosms over three months. This might have counteracted the shading of submerged macrophytes provided by phytoplankton and periphyton. The results of the experiment are consistent with observations of higher macrophyte resilience to nutrient loading in Mediterranean lakes compared with northern temperate lakes.     

Species richness,composition, and abundance of fish larvae and juveniles inhabiting natural and developed shorelines of a glacial Iowa lake

Synopsis Young-of-the-year fish communities in naturally vegetated sites were compared with those inhabiting nearby sites where lakeshore development (i.e., construction of homes, boat docks, and beaches) reduced nearshore macrophyte species richness and abundance. The study was conducted in a 2266 hectare, glacially formed, eutrophic lake in northwestern Iowa during the summers of 1987 and 1988. Study sites were divided into 3 depth zones, and fishes were collected by seining (0–1 m), plexiglass traps (1–2 m), and a nonclosing Tucker trawl (2–3 m). Species richness and total fish abundance were consistently greater in natural than in developed sites in both nearshore (0–1 m) and intermediate (1–2 m) depth zones, but differed little between natural and developed sites in the offshore (2–3 m) depth zone. Nearly 50% of the species sampled, including yellow perch Perca flavescens and bluegill Lepomis macrochirus, inhabited limnetic areas as larvae before migrating inshore as juveniles. Eighteen of the 20 fish species collected as juveniles were in greater abundance in natural than in developed sites. Smallmouth bass Micropterus dolomieui was the only game species consistently found in equal or greater abundance in developed sites. Within all sites, juvenile fishes were generally most abundant where macrophyte abundance and species richness were greatest. Findings from this study demonstrate the importance of nearshore aquatic vegetation to fishes during their first summer of life. If nearshore vegetation beds of lakes continue to be regarded as a nuisance and indiscriminately removed, important fish nursery habitat will be lost. The short-term result will likely be reduced year-class strength of vegetation-dependent species. More importantly, the long-term effects will be changes in fish community richness and composition which will, in turn, alter the lake's fishery.