Abundance Of Planktivorous Fish Research Articles

Transition to Piscivory Seen Through Brain Transcriptomics in a Juvenile Percid Fish: Complex Interplay of Differential Gene Transcription, Alternative Splicing, and ncRNA Activity.

Pikeperch (Sander Lucioperca) belongs to main predatory fish species in freshwater bodies throughout Europe playing the key role by reducing planktivorous fish abundance. Two size classes of the young-of-the-year (YOY) pikeperch are known in Europe and North America. Our long-term fish survey elucidates late-summer size distribution of YOY pikeperch in the Lipno Reservoir (Czechia) and recognizes two distinct subcohorts: smaller pelagic planktivores heavily outnumber larger demersal piscivores. To explore molecular mechanisms accompanying the switch from planktivory to piscivory, we compared brain transcriptomes of both subcohorts and identified 148 differentially transcribed genes. The pathway enrichment analyses identified the piscivorous phase to be associated with genes involved in collagen and extracellular matrix generation with numerous Gene Ontology (GO), while the planktivorous phase was associated with genes for non-muscle-myosins (NMM) with less GO terms. Transcripts further upregulated in planktivores from the periphery of the NMM network were Pmchl, Pomcl, and Pyyb, all involved also in appetite control and producing (an)orexigenic neuropeptides. Noncoding RNAs were upregulated in transcriptomes of planktivores including three transcripts of snoRNA U85. Thirty genes mostly functionally unrelated to those differentially transcribed were alternatively spliced between the subcohorts. Our results indicate planktivores as potentially driven by voracity to initiate the switch to piscivory, while piscivores undergo a dynamic brain development. We propose a spatiotemporal spreading of juvenile development over a longer period and larger spatial scales through developmental plasticity as an adaptation to exploiting all types of resources and decreasing the intraspecific competition.

Cleaner Fish Labroides dimidiatus Presence Does Not Indirectly Affect Demersal Zooplankton

Coral reef mutualisms involve complex trophic ecological relationships that produce indirect effects. Excluding mutualistic cleaner fish Labroides dimidiatus from reefs indirectly increases the abundance of many fishes and reduces demersal stages of their ectoparasitic prey (gnathiid isopods). Whether cleaners affect populations of planktivorous fishes that consume demersal zooplankton, and consequently indirectly affect the rest of the demersal zooplankton community — via presumed changes in planktivory — is unknown. Therefore, using a long-term cleaner fish manipulation on patch reefs (July 2000 to December 2012, Great Barrier Reef), we tested whether cleaner treatment (removal or control) affects planktivorous fish abundance and diversity, and demersal zooplankton biomass, abundance, and diversity. Fish surveys, 9 and 12 years after removing cleaners revealed fewer fish on removal compared to control reefs for one of the three most abundant planktivores, but not total abundance (Pomacentridae, 26 species), diversity, and composition. Emerging zooplankton were sampled during the day and night over nine sampling trips across 12 years. There was no effect of cleaner treatment on post-removal values, compared with pre-removal values in July 2000, for zooplankton biomass, abundance, diversity, and composition (34 taxa). Zooplankton abundance showed no diel differences, but diversity, and occasionally biomass, were higher at night. Zooplankton composition also showed diel differences, with three taxa contributing the most to this dissimilarity [Cirripeda nauplii, Facetotecta (Arthopoda), Oikopleuridae (Chordata)]. Zooplankton diversity did not differ among times, abundance was higher in January 2002 relative to July 2000, and composition differed among all times. The lack of detectable indirect effects of cleaner fish presence on zooplankton (non-gnathiid) may partly be due to cleaners’ variable effect on planktivorous fish abundance, but also the result of invertebrate planktivory and other processes that affect zooplankton populations not investigated here. Nevertheless, the pronounced diel and temporal changes in zooplankton observed likely influence coral reef trophic interactions.

Association behavior between sand tiger sharks and round scad is driven by mesopredators.

In marine systems, behaviorally-mediated indirect interactions between prey, mesopredators, and higher trophic-level, large predators are less commonly investigated than other ecologic interactions, likely because of inherent difficulties associated with making observations. Underwater videos (n = 216) from SharkCam, a camera installation sited beneath Frying Pan Tower, a decommissioned light house and platform, on a natural, hard bottom site approximately 50 km off Cape Fear, North Carolina, were used to investigate association behavior of round scad Decapterus punctatus around sand tiger sharks Carcharias taurus. Videos containing sand tiger sharks were analyzed for the simultaneous presence of round scad, and six species of scad mesopredators, with scad-shark interactions assigned to one of three categories of association: no visible interaction, loosely associated, or tightly associated. The likelihood of scad being loosely or tightly associated with sharks was significantly higher in the presence of scad mesopredators, suggesting that sharks provide a predation refuge for scad. This behaviorally-mediated indirect interaction has important implications for trophic energy transfer and mesopredator control on hard bottoms, as scad are one of the most abundant planktivorous fish on hard bottoms in the western Atlantic Ocean. Although we were not able to provide statistical evidence that sand tiger sharks also benefit from this association behavior, we have clear video evidence that round scad association conceals and attracts mesopredators, enhancing predation opportunities for sand tiger sharks. These interactions potentially yield additional trophic consequences to this unique association and highlight the value of exploring behaviorally-mediated interactions in marine communities.

Diel changes in vertical and horizontal distribution of cladocerans in two deep lakes during early and late summer

Temporal and spatial heterogeneity in the distribution of cladocerans in lakes could be caused by abiotic (wind, water currents) and biotic factors (reproduction, food resources, predation). Diel horizontal and vertical distribution of cladoceran assemblages was studied in two deep lakes (Milada and Most Lakes, Czech Republic) in early (June) and late (September) summer. The objective was to study diel vertical migration (DVM) and diel horizontal migration (DHM) of cladocerans under conditions of different macrophyte cover in littoral areas (rich in Milada Lake, poor in Most Lake) and fish assemblages (non-specialised planktivorous species in Milada Lake, and abundant planktivorous fish - maraena whitefish, Coregonus maraena - in open water habitats in Most Lake). Temporal variations in cladoceran assemblages were reported in both lakes in the two sampling periods. DVM was observed in the two lakes, performed by the most vulnerable species to fish predation (the larger Daphnia spp.), but with different patterns (direct and reverse) probably linked with the local fish community and other biotic and abiotic factors in each lake. Horizontal movements were only observed in Most Lake: D. longispina increase its abundance in open waters at night compared to the littoral points; while Ceriodaphnia spp. showed the inverse pattern. In both lakes, higher densities were often found at night in surface layers, producing a great “diurnal deficit”: cladocerans remain undetected in some zones during the day (especially in the littoral areas) moving to surface layers at night.

The CO2 -equivalent balance of freshwater ecosystems is non-linearly related to productivity.

Eutrophication of fresh waters results in increased CO2 uptake by primary production, but at the same time increased emissions of CH4 to the atmosphere. Given the contrasting effects of CO2 uptake and CH4 release, the net effect of eutrophication on the CO2 -equivalent balance of fresh waters is not clear. We measured carbon fluxes (CO2 and CH4 diffusion, CH4 ebullition) and CH4 oxidation in 20 freshwater mesocosms with 10 different nutrient concentrations (total phosphorus range: mesotrophic 39µg/L until hypereutrophic 939µg/L) and planktivorous fish in half of them. We found that the CO2 -equivalent balance had a U-shaped relationship with productivity, up to a threshold in hypereutrophic systems. CO2 -equivalent sinks were confined to a narrow range of net ecosystem production (NEP) between 5 and 19mmolO2 m-3 day-1 . Our findings indicate that eutrophication can shift fresh waters from sources to sinks of CO2 -equivalents due to enhanced CO2 uptake, but continued eutrophication enhances CH4 emission and transforms freshwater ecosystems to net sources of CO2 -equivalents to the atmosphere. Nutrient enrichment but also planktivorous fish presence increased productivity, thereby regulating the resulting CO2 -equivalent balance. Increasing planktivorous fish abundance, often concomitant with eutrophication, will consequently likely affect the CO2 -equivalent balance of fresh waters.

Climate effects on temporal and spatial dynamics of phytoplankton and zooplankton in the Barents Sea

Temporal and spatial dynamics of phytoplankton and zooplankton in the Barents Sea have been investigated during the last three decades using remote sensing and in situ observations. Satellite-derived sea surface temperatures increased in the period 1998–2017 by 1.0 °C as an average for the Barents Sea. We found significant positive relationships between ice-free conditions (open water area and duration) and satellite-based net primary production (NPP). The estimated annual NPP for the Barents Sea more than doubled over the 1998–2017 period, from around 40 to over 100 Tg C. The strong increase in NPP is the result of reduction of sea ice, extending both the area and period available for phytoplankton production. In areas where ice extent has decreased, satellite-derived chlorophyll a shows that the timing of the peak spring phytoplankton bloom has advanced by over a month. Our results reveal that phytoplankton dynamics in the ecosystem have been changing rapidly and that this change is driven mainly by bottom-up climatic processes. Autumn mesozooplankton biomass showed strong interannual variability in the 1990s, displaying an inverse relationship with capelin biomass, the most abundant planktivorous fish. In some regions, e.g. Central Bank, capelin biomass explained up to 50% of the mesozooplankton variability during 1989–2017. Though capelin biomass has varied considerably, mesozooplankton biomass has remained rather stable since the mid-2000s (6–8 g dry wt. m−2), resulting in a weakening of the negative relationship between capelin and mesozooplankton biomass in recent years. The stable zooplankton biomass indicates favorable conditions (prolonged/increased NPP) for mesozooplankton production, partly counteracting high predation levels. Overall, we observed trends in phytoplankton phenology that were strongly associated with changes in sea ice cover driven by fluctuations in temperature regime, a trend that may intensify should the ecosystem become even warmer due to climate change. Further reductions of sea ice and associated ice algae is expected to have adverse effects on sympagic fauna and ice dependent species in the Arctic food web. The ice-free conditions may promote further Atlantification (or borealization) of plankton and fish communities in the Barents Sea.

Deficiency syndromes in top predators associated with large-scale changes in the Baltic Sea ecosystem.

Vitamin B1 (thiamin) deficiency is an issue periodically affecting a wide range of taxa worldwide. In aquatic pelagic systems, thiamin is mainly produced by bacteria and phytoplankton and is transferred to fish and birds via zooplankton, but there is no general consensus on when or why this transfer is disrupted. We focus on the occurrence in salmon (Salmo salar) of a thiamin deficiency syndrome (M74), the incidence of which is highly correlated among populations derived from different spawning rivers. Here, we show that M74 in salmon is associated with certain large-scale abiotic changes in the main common feeding area of salmon in the southern Baltic Sea. Years with high M74 incidence were characterized by stagnant periods with relatively low salinity and phosphate and silicate concentrations but high total nitrogen. Consequently, there were major changes in phytoplankton and zooplankton, with, e.g., increased abundances of Cryptophyceae, Dinophyceae, Diatomophyceae and Euglenophyceae and Acartia spp. during high M74 incidence years. The prey fish communities also had increased stocks of both herring and sprat in these years. Overall, this suggests important changes in the entire food web structure and nutritional pathways in the common feeding period during high M74 incidence years. Previous research has emphasized the importance of the abundance of planktivorous fish for the occurrence of M74. By using this 27-year time series, we expand this analysis to the entire ecosystem and discuss potential mechanisms inducing thiamin deficiency in salmon.

Modeling vitamin B1 transfer to consumers in the aquatic food web

Vitamin B1 is an essential exogenous micronutrient for animals. Mass death and reproductive failure in top aquatic consumers caused by vitamin B1 deficiency is an emerging conservation issue in Northern hemisphere aquatic ecosystems. We present for the first time a model that identifies conditions responsible for the constrained flow of vitamin B1 from unicellular organisms to planktivorous fishes. The flow of vitamin B1 through the food web is constrained under anthropogenic pressures of increased nutrient input and, driven by climatic change, increased light attenuation by dissolved substances transported to marine coastal systems. Fishing pressure on piscivorous fish, through increased abundance of planktivorous fish that overexploit mesozooplankton, may further constrain vitamin B1 flow from producers to consumers. We also found that key ecological contributors to the constrained flow of vitamin B1 are a low mesozooplankton biomass, picoalgae prevailing among primary producers and low fluctuations of population numbers of planktonic organisms.

Eutrophication mediates rapid clonal evolution in Daphnia pulicaria

Abstract Laboratory studies have revealed that Daphnia species can evolve to tolerate toxic cyanobacteria in the diet. Specifically, Daphnia from eutrophic lakes where cyanobacteria are common tend to have higher growth rates and survival when fed toxic cyanobacteria than populations from oligotrophic environments with low abundance of cyanobacteria. We conducted an in‐lake mesocosm (i.e. limnocorral) experiment during the autumn of 2009 to assess the effects of nutrient enrichment on clonal evolution in Daphnia pulicaria. As nutrient enrichment often favours grazing‐resistant cyanobacteria, we hypothesised that fertilisation would influence the genotypic composition of D. pulicaria that vary in tolerance to cyanobacteria. Mesocosms were fertilised to manipulate phytoplankton and cyanobacterial abundance and concentrations of a cyanobacterial toxin (microcystin). Thus, half of the mesocosms were high‐nutrient and half were low‐nutrient. We then stocked half of the mesocosms with a mixture of six genetically‐distinct D. pulicaria genotypes (three genotypes from oligotrophic lakes and three from eutrophic lakes) leaving half of the mesocosms Daphnia‐free to assess grazing effects, using a fully factorial design. When compared to the low nutrient treatment, high nutrient mesocosms had nearly five‐fold higher chlorophyll a concentrations, eight‐fold higher cyanobacterial dry biomass, and three‐fold higher microcystin levels at the start of the experiment. In contrast, low nutrient mesocosms had phytoplankton concentrations typical of mesotrophic lakes. Fertilisation strongly affected Daphnia genetic diversity in the mesocosms. Final Daphnia genotype diversity in the mesocosms with low‐cyanobacteria (richness = 5.83, Shannon–Weiner index = 1.55, evenness = 0.88) was similar to the initial stocked diversity (richness = 5.50, Shannon–Weiner index = 1.48, evenness = 0.87). In contrast, final diversity in fertilised mesocosms with high cyanobacteria was greatly reduced (richness = 2, Shannon–Weiner index = 0.17), with one Daphnia genotype that originated from the most‐eutrophic lake being highly dominant (evenness = 0.25). Thus, eutrophication mediated strong clonal selection of a cyanobacteria‐tolerant Daphnia genotype over just 10 weeks. By the end of the experiment, Daphnia significantly reduced phytoplankton biomass in the high‐nutrient, but not in the low‐nutrient treatment. This difference in effect size was largely driven by the five‐fold higher initial phytoplankton biomass in the high‐nutrient treatment. Thus, the ability of Daphnia to reduce phytoplankton biomass in eutrophic lakes may be driven more so by the abundance of planktivorous fishes, as opposed to the prevalence of cyanobacteria and their associated toxins.

Effect of non-native versus native invaders on macrophyte richness: are carp and bullheads ecological proxies?

While it is accepted that invasive species are non-native organisms that become abundant and cause ecological damage in areas where they are introduced, the problem of ‘native invaders,’ native species that become excessively abundant due to anthropogenic impacts, is frequently encountered by ecologists. Often, native and non-native invaders occur in sympatry. Understanding relative severity of their impacts and niches they occupy is needed to inform management actions. Here, we quantify relative impact of native (black bullhead) and non-native (common carp) benthic fish on macrophytes species richness in over 200 lakes in North America. The impact of each species was addressed while accounting for the effects of water clarity, depth, lake area, watershed size, shoreline irregularity, land use by humans, abundance of planktivorous fishes, and ecoregion. Using model selection, we show that both species had negative impact on macrophytes richness, but the impact of carp was approximately two times as strong when adjusted for catch rates. We also conducted a principal component analysis followed by permutation procedures, which showed that carp and bullheads often occurred together in shallow, turbid lakes in watershed dominated by human use. Our findings have implications for lake-restoration efforts via carp or bullhead management.

Fish shift the feeding behaviour and trophic niche diversification of their prey in subarctic Lake Mývatn, Iceland

Fish can alter food web structure through trophic cascades. While most studies conducted in oligotrophic subarctic lakes show strong top–down control on consumers in the presence of fish, several studies undertaken in eutrophic subarctic Lake Mývatn, Iceland, suggest that it is consumer–resource interactions that drive the whole-lake community. Here, we used stable isotopes of carbon and nitrogen from the main food web compartments derived from a 3-month in situ-controlled mesocosm experiment involving two treatments (with and without fish) with three replicates each to determine the effects of fish on the trophic structure of the Lake Mývatn food web. We found that the whole food web trophic structure shifted towards the upper part of the water column (more planktonic habitat) in fishless enclosures. Additionally, the trophic niche of organisms occupying the base of the food web became more diversified when fish were absent, and the trophic redundancy of all taxa decreased (more dissimilar trophic niches). Stronger top–down effects may also result from global warming, producing increased abundance of planktivorous fish in subarctic lakes. Our results indicate that this could lead to a shift in trophic niche and reduced trophic diversity of most food web organisms.

Prey selection of larval and juvenile planktivorous fish: impacts of introduced prey

Changes in the abundance and composition of zooplankton resulting from species introductions may be contributing to a decline in the abundance of planktivorous fish in the San Francisco Estuary (SFE). To examine how changes in the prey assemblage may translate into feeding success of planktivorous fish, we quantified the relative consumption of native and introduced copepods by larval and juvenile delta smelt, Hypomesus transpacificus, and larval striped bass, Morone saxatilis, in laboratory feeding experiments. Larvae ≤20 days post-hatch (dph) of both fish consumed the copepodite stages of ~1 mm calanoid copepods, native Eurytemora affinis and introduced Pseudodiaptomus forbesi, and the smaller (~0.5 mm) introduced cyclopoid copepod, Limnoithona tetraspina, in approximately the same proportion as their abundance in the assemblage. Conversely, prey selection of larvae >20 dph shifted towards the larger calanoid species, E. affinis and P. forbesi. The timing of this shift differed between fish species, occurring earlier in striped bass, and varied between calanoid species, occurring later for P. forbesi than E. affinis. These data suggest that the impacts of introduced prey on planktivorous fish depend not only on the prey being introduced, but also on the resident predators. Additionally, even small differences in size and behavior between native and introduced prey can influence prey selection of planktivorous fish. Changes in prey selection of planktivorous fish resulting from species introductions could directly impact fish recruitment. As a result, species introductions could significantly alter food web function in the SFE and other invaded aquatic ecosystems.

An assessment of the ecological potential of Central and Western European reservoirs based on fish communities

In this study we developed a novel methodology based on fish communities to assess the ecological potential of central European reservoirs. Using the hindcasting approach, our index predicts values that could be observed in the absence of pressures for each reservoir depending on their environmental characteristics. Fish data were collected from 144 French and Czech reservoirs between 2005 and 2013 by standardized benthic gillnet sampling and transformed to functional and taxonomical metrics. After all validation by multiple testing of models redundancy and pressure-response, the final index was composed of three metrics: total biomass of fish, abundance of invertivores/piscivores, and abundance of planktivorous fish. The index accurately identifies reservoirs that are lightly, moderately and heavily affected by eutrophication. In addition to French and Czech reservoirs, this index could be a useful tool for countries with few reservoirs and the basis for further collaborative studies.

Lake restoration by biomanipulation using piscivore and Daphnia stocking; results of the biomanipulation in Japan

Biomanipulation has been employed in numerous locations throughout the world as a means for reducing phytoplankton biomass; however, it has not been employed very often in Japan. A common approach involves the introduction of piscivorous fish to reduce the abundance of planktivorous fish. In our study, to first apply biomanipulation, we stocked Lake Shirakaba (a high-altitude, protected area in a park) in central Japan with rainbow trout fingerlings and cladoceran Daphnia (Daphnia galeata) in 2000. A “pre-biomanipulation” data set (1997–1999) and “a post-biomanipulation” data set (2000–2006) allowed us to evaluate the lake's response to biomanipulation. After the biomanipulation, zoo-planktivorous pond smelt disappeared and a large population of Daphnia had been established, which substantially reduced the number of the previously dominant small cladocerans and rotifers. Water transparency increased from about 2 m (before biomanipulation) to more than 4 m (after biomanipulation). Reductions in algal biomass and increased transparency led to expansion of the submerged macrophyte Elodea nuttallii. Total phosphorus concentrations declined as well over this time period. Based on these results, we concluded that biomanipulation using piscivore and Daphnia stocking succeeded in improving lake water quality by reducing algal abundance and providing favorable conditions for the establishment of rooted plants.

Examining the effects of climate change, acidic deposition, and copper sulphate poisoning on long-term changes in cladoceran assemblages

We analyzed cladoceran remains in dated sediment cores from four lakes in Nova Scotia, Canada, to assess the potential effects of climate warming, acidic deposition, and a major fish kill caused by copper sulphate poisoning on assemblage composition and Bosmina size structure. In three of the four lakes, we observed a decline in Daphnia in the early 20th century that might be indicative of limnological changes in response to acidic deposition or increased fish predation. The appearance of the softwater zooplankter Holopedium glacialis in Hirtle Lake ~1995 might be linked to declining aqueous [Ca], a consequence of acidic deposition. No shifts in subfossil Cladocera were identified in this study that could be linked to climate warming. The application of copper sulphate as a fish poison to Trefry Lake (the lake was later re-stocked with trout fry) in 1938 resulted in dramatic and persistent changes in the cladoceran assemblage, where littoral Cladocera declined in relative abundance and small, pelagic Bosmina increased. In addition, we observed a reduction in the mean body size of Bosmina in post-1938 sediments, suggesting that planktivorous fish abundance increased and/or predatory copepods and other invertebrate predators decreased. No recovery of Cladocera to pre-disturbance conditions was observed. Overall, our data suggest that acidic deposition and calcium decline may have had a modest impact on the cladoceran communities in these lakes, but the effects of copper sulphate poisoning on the food web in Trefry Lake were widespread, and still persist over 70 years following this intervention.

Application of the daily egg production method to Baltic sprat

Baltic sprat (Sprattus sprattus balticus, Schneider 1908) is a key species in the Baltic Sea ecosystem, where it is the most abundant planktivorous fish. In the present study, we applied the daily egg production method (DEPM) for the years 1999–2008 to estimate the size of the stock component reproducing in the Bornholm Basin, a major spawning ground for sprat and cod. This is the first study assessing this stock with a fishery independent egg production method for a consecutive time series of ten years. DEPM stock size estimates were compared with those obtained by a multi species virtual population analysis for the same stock component and results from an acoustic survey. In general, the results obtained by the DEPM were in the same order of magnitude compared to the other methods and most similar to the acoustic estimate. However, in some years differences between methods were substantial. With respect to previous egg production methods to assess Baltic sprat stock components our approach takes several aspects into account which were ignored before, e.g. effect of ambient temperature range on sprat egg stage duration and mortality and interannual variability of adult stock parameters. Since the accurate determination of the daily spawning fraction bears major uncertainties, different scenarios were tested for this parameter. Least deviation compared to the other assessment methods was obtained when using a daily female spawning fraction of 24%, which corresponds well to values described in literature. The applicability of the DEPM to Baltic sprat was clearly demonstrated. Thus, it can serve as valuable tool for the estimation of Baltic sprat stock sizes independent of data obtained from commercial fisheries.

Seasonal zooplankton dynamics in Lake Michigan: Disentangling impacts of resource limitation, ecosystem engineering, and predation during a critical ecosystem transition

We examined seasonal dynamics of zooplankton at an offshore station in Lake Michigan from 1994 to 2003 and 2007 to 2008. This period saw variable weather, declines in planktivorous fish abundance, the introduction and expansion of dreissenid mussels, and a slow decline in total phosphorus concentrations. After the major expansion of mussels into deep water (2007–2008), chlorophyll in spring declined sharply, Secchi depth increased markedly in all seasons, and planktivorous fish biomass declined to record-low levels. Overlaying these dramatic ecosystem-level changes, the zooplankton community exhibited complex seasonal dynamics between 1994–2003 and 2007–2008. Phenology of the zooplankton maximum was affected by onset of thermal stratification, but there was no other discernable effect due to temperature. Interannual variability in zooplankton biomass during 1994 and 2003 was strongly driven by planktivorous fish abundance, particularly age-0 and age-1 alewives. In 2007–2008, there were large decreases in Diacyclops thomasi and Daphnia mendotae possibly caused by food limitation as well as increased predation and indirect negative effects from increases in Bythotrephes longimanus abundance and in foraging efficiency associated with increased light penetration. The Bythotrephes increase was likely driven in part by decreased predation from yearling and older alewife. While there was a major decrease in epilimnetic–metalimnetic herbivorous cladocerans in 2007–2008, there was an increase in large omnivorous and predacious calanoid copepods, especially those in the hypolimnion. Thus, changes to the zooplankton community are the result of cascading, synergistic interactions, including a shift from vertebrate to invertebrate planktivory and mussel ecosystem impacts on light climate and chlorophyll.

Internal phosphorus load estimation during biomanipulation in a large polymictic and mesotrophic lake

We quantified internal phosphorus (P) load for 26 years in the polymictic, large (155 km2) and shallow (mean depth 5.5 m) Lake Säkylän Pyhäjärvi, which was heavily biomanipulated by fish removal. Internal load was estimated as (1) partially net estimates from in situ P summer increases, (2) net estimates from P budgets (mass balance approach), and (3) gross estimates from predicted active sediment release area and sediment P release, dependent on August lake temperature. Long-term averages of these estimates were similar and large at about 60% of average external load (105 mg m−2 yr−1) and were larger than external load in years with high water temperature and low water load. Regression analysis revealed that external load is decreasing but internal load is increasing over time. Internal load was negatively correlated with annual water load and positively correlated with lake water temperature. Long-term average annual or summer P concentrations are adequately predicted by a P mass balance model that includes external load, internal load (Method 3), and sedimentation as independently predicted retention. Predictability was poor for individual years, however, partially due to the poor correlation of observed lake and outflow P concentrations, the variable abundance of planktivorous fish, and reflecting the violation of the steady state assumption when individual years are modeled. Scenario modeling shows that biomanipulation cancels out the effects of internal load and forecasts a rapid increase of internal load and P concentration due to climate change; therefore, measures that further decrease external and internal P load and strengthen biomanipulation are recommended.

Ecological and socio‐economic impacts of invasive water hyacinth (Eichhornia crassipes): a review

Summary1. Water hyacinth (Eichhornia crassipes) is one of the world’s most invasive aquatic plants and is known to cause significant ecological and socio‐economic effects.2. Water hyacinth can alter water clarity and decrease phytoplankton production, dissolved oxygen, nitrogen, phosphorous, heavy metals and concentrations of other contaminants.3. The effects of water hyacinth on ecological communities appear to be largely nonlinear. Abundance and diversity of aquatic invertebrates generally increase in response to increased habitat heterogeneity and structural complexity provided by water hyacinth but decrease due to decreased phytoplankton (food) availability.4. Effects of water hyacinth on fish are largely dependent on original community composition and food‐web structure. A more diverse and abundant epiphytic invertebrate community may increase fish abundance and diversity, but a decrease in phytoplankton may decrease dissolved oxygen concentrations and planktivorous fish abundance, subsequently affecting higher trophic levels.5. Little is known about the effects of water hyacinth on waterbird communities; however, increases in macroinvertebrate and fish abundance and diversity suggest a potentially positive interaction with waterbirds when water hyacinth is at moderate density.6. The socio‐economic effects of water hyacinth are dependent on the extent of the invasion, the uses of the impacted waterbody, control methods and the response to control efforts. Ecosystem‐level research programmes that simultaneously monitor the effects of water hyacinth on multiple trophic‐levels are needed to further our understanding of invasive species.

Spatial distribution of the Daphnia longispina species complex and other planktonic crustaceans in the heterogeneous environment of canyon-shaped reservoirs

Canyon-shaped reservoirs are often characterised by longitudinal gradients of environmental factors (including trophic level, phytoplankton and zooplankton biomass and abundance of planktivorous fish) affecting the taxonomic composition of the pelagic community. We tested the hypothesis that the spatial distribution of different species and interspecific hybrids of the Daphnia longispina species complex is non-random under such conditions. During the summer stratification, we sampled crustacean zooplankton from 11 reservoirs, covering both longitudinal (upstream, middle, dam) and vertical (epi-, meta- and hypolimnion) environmental gradients. Allozyme electrophoresis was used to discriminate among different Daphnia taxa. All three frequently hybridizing European species of the complex (galeata, cucullata, longispina = hyalina) and hybrids with Daphnia galeata were commonly recorded. Smaller-bodied Daphnia cucullata and its hybrids, when present, preferred mostly the nutrient- and food-rich upstream regions; D. longispina and its hybrids were more commonly found in the downstream part, and often dominated in the meta- or hypolimnion. Redundancy analyses confirmed significant differences in the Daphnia taxon composition, as well as in spatial distribution of other crustacean species, along both gradients. For the first time, we demonstrate regular patterns in the horizontal distribution of Daphnia species and hybrids within a water body, thus accepting our hypothesis. Such spatial distributional patterns may strongly impact local hybridization processes.