Rikke Bjerring

Impacts of climate warming on lake fish community structure and potential effects on ecosystem function

Fish play a key role in the trophic dynamics of lakes, not least in shallow systems. With climate warming, complex changes in fish community structure may be expected owing to the direct and indirect effects of temperature, and indirect effects of eutrophication, water-level changes and salinisation on fish metabolism, biotic interactions and geographical distribution. We review published and new data supporting the hypotheses that, with a warming climate, there will be changes in: fish community structure (e.g. higher or lower richness depending on local conditions); life history traits (e.g. smaller body size, shorter life span, earlier and less synchronised reproduction); feeding mode (i.e. increased omnivory and herbivory); behaviour (i.e. stronger association with littoral areas and a greater proportion of benthivores); and winter survival. All these changes imply higher predation on zooplankton and macroinvertebrates with increasing temperatures, suggesting that the changes in the fish communities partly resemble, and may intensify, the effects triggered by eutrophication. Modulating factors identified in cold and temperate systems, such as the presence of submerged plants and winter ice cover, seem to be weaker or non-existent in warm(ing) lakes. Consequently, in the future lower nutrient thresholds may be needed to obtain clear-water conditions and good ecological status in the future in currently cold or temperate lakes. Although examples are still scarce and more research is needed, we foresee biomanipulation to be a less successful restoration tool in warm(ing) lakes without a strong reduction of the nutrient load.

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

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

Biomanipulation as a Restoration Tool to Combat Eutrophication: Recent Advances and Future Challenges

Eutrophication resulting from high nutrient loading has been the paramount environmental problem for lakes world-wide for the past four decades. Efforts are being made in many parts of the world to reduce external nutrient loading via improved wastewater treatment or diversion of nutrient-rich inflows. However, even after a reduction of the external phosphorus loading, the effects obtained may be unsatisfactory. This may reflect an insufficient reduction in the external nutrient loading to effectively limit phytoplankton growth. However, the lack of success may also be due to chemical or biological within-lake inertia preventing or delaying improvements. To overcome the resilience and thereby reinforce recovery, a number of physico-chemical and biological restoration methods have been developed. In this chapter, we describe recent developments of biological restoration methods related to eutrophication, their short-term and long-term effects, and discuss the possibility of using combined physico-chemical and biological methods to improve the long-term stability of restoration and to reduce restoration costs. As comprehensive reviews of the effect of fish manipulation in cold temperate lakes are numerous, for these waterbodies, we highlight recent results, including effects on biodiversity and metabolism, and present new approaches of biomanipulation. Our particular focus is, however, directed at biomanipulation in warm lakes and on combined treatments which are far less well described in the literature.

A community-based framework for aquatic ecosystem models

Here, we communicate a point of departure in the development of aquatic ecosystem models, namely a new community-based framework, which supports an enhanced and transparent union between the collective expertise that exists in the communities of traditional ecologists and model developers. Through a literature survey, we document the growing importance of numerical aquatic ecosystem models while also noting the difficulties, up until now, of the aquatic scientific community to make significant advances in these models during the past two decades. Through a common forum for aquatic ecosystem modellers we aim to (i) advance collaboration within the aquatic ecosystem modelling community, (ii) enable increased use of models for research, policy and ecosystem-based management, (iii) facilitate a collective framework using common (standardised) code to ensure that model development is incremental, (iv) increase the transparency of model structure, assumptions and techniques, (v) achieve a greater understanding of aquatic ecosystem functioning, (vi) increase the reliability of predictions by aquatic ecosystem models, (vii) stimulate model inter-comparisons including differing model approaches, and (viii) avoid ‘re-inventing the wheel’, thus accelerating improvements to aquatic ecosystem models. We intend to achieve this as a community that fosters interactions amongst ecologists and model developers. Further, we outline scientific topics recently articulated by the scientific community, which lend themselves well to being addressed by integrative modelling approaches and serve to motivate the progress and implementation of an open source model framework.

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

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

Effects of climate and nutrient load on the water quality of shallow lakes assessed through ensemble runs by PCLake

Complex ecological models are used to predict the consequences of anticipated future changes in climate and nutrient loading for lake water quality. These models may, however, suffer from nonuniqueness in that various sets of model parameter values may yield equally satisfactory representations of the system being modeled, but when applied in future scenarios these sets of values may divert considerably in their simulated outcomes. Compilation of an ensemble of model runs allows us to account for simulation variability arising from model parameter estimates. Thus, we propose a new approach for aquatic ecological models creating a more robust prediction of future water quality. We used our ensemble approach in an application of the widely used PCLake model for Danish shallow Lake Arreskov, which during the past two decades has demonstrated frequent shifts between turbid and clear water states. Despite marked variability, the span of our ensemble runs encapsulated 70–90% of the observed variation in lake water quality. The model exercise demonstrates that future warming and increased nutrient loading lead to lower probability of a clear water, vegetation-rich state and greater likelihood of cyanobacteria dominance. In a 6.0°C warming scenario, for instance, the current nutrient loading of nitrogen and phosphorus must be reduced by about 75% to maintain the present ecological state of Lake Arreskov, but even in a near-future 2.0°C warming scenario, a higher probability of a turbid, cyanobacteria-dominated state is predicted. As managers may wish to determine the probability of achieving a certain ecological state, our proposed ensemble approach facilitates new ways of communicating future stressor impacts.

Cross‐taxon congruence in lake plankton largely independent of environmental gradients

Groups of organisms often have congruent patterns of diversity or community structure due to similar environmental requirements. However, ecological interactions across trophic levels may also promote congruence independent of environmental drivers through selective predation, niche partitioning, or facilitation. We examined congruence between phytoplankton and zooplankton communities using 20 years of monitoring data from 17 Danish lakes, most of which were subject to external nutrient loading reduction after a period of eutrophication. Linear mixed effect models and partial Mantel tests were used to elucidate the extent to which congruence in genus richness and composition was driven by environmental factors. Congruence not explained by environmental controls might indicate ecological interactions across trophic levels of lake plankton. Genus richness and composition of phyto- and zooplankton were significantly congruent. Environmental factors had limited power to explain the genus richness of phyto- and zooplankton (R2: 6% and 12%, respectively). Including richness of the reciprocal group among the predictors markedly improved each respective model and explanatory power (R2: 11% and 17%, respectively). Similarly, a large proportion of the congruence in genus composition was independent of environmental dissimilarity (Mantel r: 0.29 and partial Mantel r after accounting for environmental control: 0.17). The strength of the congruence varied among different groups of phyto- and zooplankton, likely reflecting that the different zooplankton groups differ in their grazing size spectra and thus in the strength of their coupling with the phytoplankton assemblages. Remarkably, congruence with phytoplankton was strongest for the smallest group of zooplankton, rotifers, which graze on a limited size spectrum of phytoplankton. Moreover, congruence was stronger in low-nutrient lakes, suggesting that the strength of the interactions between the two groups weakens with eutrophication. This is likely due to changes in trophic dynamics, where enhanced fish predation on large-bodied zooplankton with increasing nutrient levels results in reduced zooplankton grazing control of phytoplankton assemblages.

Short-and long term niche segregation and individual specialization of brown trout (Salmo trutta) in species poor Faroese lakes

Trophic niche divergence is considered to be a major process by which species coexistence is facilitated. When studying niche segregation in lake ecosystems, we tend to view the niche on a one-dimensional pelagic-littoral axis. In reality, however, the niche use may be more complex and individual fidelity to a niche may be variable both between and within populations. In order to study this complexity, relative simple systems with few species are needed. In this paper, we study how competitor presence affects the resource use of brown trout (Salmo trutta) in 11 species-poor Faroese lakes by comparing relative abundance, stable isotope ratios and diet in multiple habitats. In the presence of three-spined sticklebacks (Gasterosteus aculeatus), a higher proportion of the trout population was found in the pelagic habitat, and trout in general relied on a more pelagic diet base as compared to trout living in allopatry or in sympatry with Arctic charr (Salvelinus alpinus). Diet analyses revealed, however, that niche-segregation may be more complex than described on a one-dimensional pelagic-littoral axis. Trout from both littoral and offshore benthic habitats had in the presence of sticklebacks a less benthic diet as compared to trout living in allopatry or in sympatry with charr. Furthermore, we found individual habitat specialization between littoral/benthic and pelagic trout in deep lakes. Hence, our findings indicate that for trout populations interspecific competition can drive shifts in both habitat and niche use, but at the same time they illustrate the complexity of the ecological niche in freshwater ecosystems.