Stephan Hülsmann

The impact of climate change on lakes in the Netherlands: a review

Climate change will alter freshwater ecosystems but specific effects will vary among regions and the type of water body. Here, we give an integrative review of the observed and predicted impacts of climate change on shallow lakes in the Netherlands and put these impacts in an international perspective. Most of these lakes are man-made and have preset water levels and poorly developed littoral zones. Relevant climatic factors for these ecosystems are temperature, ice-cover and wind. Secondary factors affected by climate include nutrient loading, residence time and water levels. We reviewed the relevant literature in order to assess the impact of climate change on these lakes. We focussed on six management objectives as bioindicators for the functioning of these ecosystems: target species, nuisance species, invading species, transparency, carrying capacity and biodiversity. We conclude that climate change will likely (i) reduce the numbers of several target species of birds; (ii) favour and stabilize cyanobacterial dominance in phytoplankton communities; (iii) cause more serious incidents of botulism among waterfowl and enhance the spreading of mosquito borne diseases; (iv) benefit invaders originating from the Ponto-Caspian region; (v) stabilize turbid, phytoplankton-dominated systems, thus counteracting restoration measures; (vi) destabilize macrophyte-dominated clear-water lakes; (vii) increase the carrying capacity of primary producers, especially phytoplankton, thus mimicking eutrophication; (viii) affect higher trophic levels as a result of enhanced primary production; (ix) have a negative impact on biodiversity which is linked to the clear water state; (x) affect biodiversity by changing the disturbance regime. Water managers can counteract these developments by reduction of nutrient loading, development of the littoral zone, compartmentalization of lakes and fisheries management.

Challenges and Opportunities for Integrating Lake Ecosystem Modelling Approaches

A large number and wide variety of lake ecosystem models have been developed and published during the past four decades. We identify two challenges for making further progress in this field. One such challenge is to avoid developing more models largely following the concept of others (‘reinventing the wheel’). The other challenge is to avoid focusing on only one type of model, while ignoring new and diverse approaches that have become available (‘having tunnel vision’). In this paper, we aim at improving the awareness of existing models and knowledge of concurrent approaches in lake ecosystem modelling, without covering all possible model tools and avenues. First, we present a broad variety of modelling approaches. To illustrate these approaches, we give brief descriptions of rather arbitrarily selected sets of specific models. We deal with static models (steady state and regression models), complex dynamic models (CAEDYM, CE-QUAL-W2, Delft 3D-ECO, LakeMab, LakeWeb, MyLake, PCLake, PROTECH, SALMO), structurally dynamic models and minimal dynamic models. We also discuss a group of approaches that could all be classified as individual based: super-individual models (Piscator, Charisma), physiologically structured models, stage-structured models and trait-based models. We briefly mention genetic algorithms, neural networks, Kalman filters and fuzzy logic. Thereafter, we zoom in, as an in-depth example, on the multi-decadal development and application of the lake ecosystem model PCLake and related models (PCLake Metamodel, Lake Shira Model, IPH-TRIM3D-PCLake). In the discussion, we argue that while the historical development of each approach and model is understandable given its ‘leading principle’, there are many opportunities for combining approaches. We take the point of view that a single ‘right’ approach does not exist and should not be strived for. Instead, multiple modelling approaches, applied concurrently to a given problem, can help develop an integrative view on the functioning of lake ecosystems. We end with a set of specific recommendations that may be of help in the further development of lake ecosystem models.

Can overwintering versus diapausing strategy in Daphnia determine match-mismatch events in zooplankton-algae interactions

Mismatches between predator and prey due to climate change have now been documented for a number of systems. Ultimately, a mismatch may have far-reaching consequences for ecosystem functioning as decoupling of trophic relationships results in trophic cascades. Here, we examine the potential for climate change induced mismatches between zooplankton and algae during spring succession, with a focus on Daphnia and its algal food. Whereas the development of an overwintering population of daphnids may parallel shifts in phytoplankton phenology due to climate warming, changes in the photoperiod–temperature interaction may cause the emerging population of daphnids to hatch too late and mismatch their phytoplankton prey. A decoupling of the trophic relationship between the keystone herbivore Daphnia and its algal prey can result in the absence of a spring clear water phase. We extended an existing minimal model of seasonal dynamics of Daphnia and algae and varied the way the Daphnia population is started in spring, i.e., from free swimming individuals or from hatching resting eggs. Our model results show that temperature affects the timing of peak abundance in Daphnia and algae, and subsequently the timing of the clear water phase. When a population is started from a small inoculum of hatching resting eggs, extreme climate warming (+6°C) results in a decoupling of trophic relationships and the clear water phase fails to occur. In the other scenarios, the trophic relationships between Daphnia and its algal food source remain intact. Analysis of 36 temperate lakes showed that shallow lakes have a higher potential for climate induced match–mismatches, as the probability of active overwintering daphnids decreases with lake depth. Future research should point out whether lake depth is a direct causal factor in determining the presence of active overwintering daphnids or merely indicative for underlying causal factors such as fish predation and macrophyte cover.

Initiation of the midsummer decline of Daphnia as related to predation, non-consumptive mortality and recruitment: a balance

The midsummer decline (MSD) of daphnids has been attributed to a range of factors related to either resource availability or predation. However, the relative magnitude and importance of each of these factors remained unknown. Therefore, we quantified simultaneously, but independently, both consumptive mortality due to predation and non-consumptive mortality of Daphnia galeata due to senescence, disease, food limitation as well as limiting physical or chemical factors between May and July 1998 in Bautzen Reservoir, Germany. These losses were balanced by the population gains by recruitment. The predation pressure exerted by age-0 fish, by older zooplanktivorous fish and by carnivorous zooplankton did not completely account for the observed mortality pattern of D. galeata that caused the MSD. Non-consumptive mortality in the field was calculated from recruitment patterns estimated from field samples and from age-specific survival probabilities of Daphnia offspring according to lifetable experiments. The results suggest a high non-consumptive mortality during the clear-water phase which is comparable to the magnitude of consumptive losses and may give a possible explanation for the remaining part of Daphnia mortality. By integrating all data concerning the initial abundance in early May as well as population gains and losses of daphnids, a balance sheet was drawn up for four years of investigation (1995-1998). The differences between the cumulated positive and negative book values corresponded well to the actually observed Daphnia abundance in Bautzen Reservoir at the transition period to an MSD of every year. Generally, the hypothesis is supported that an MSD should be expected if the period with bottom-up limitation during the clear-water phase overlaps strongly with top-down effects whereby in detail the following combination of conditions should be fulfilled: (i) a high biomass of age-0 percids in May/June, (ii) an early start of Daphnia consumption by carnivorous zooplankton, (iii) a quick increase in Daphnia density leading to the dominance of a peak cohort during the clear-water phase and (iv) a long lasting clear-water period characterized by reduced longevity and low recruitment of Daphnia. [KEYWORDS: AGE-0 PERCIDS; ROACH; CARNIVOROUS ZOOPLANKTON; POPULATION BUDGET; DAPHNIA; LONGEVITY]

Recruitment patterns of Daphnia: a key for understanding midsummer declines?

The population dynamics of D. galeata was studied during the period May–July of 5 successive years in Bautzen Reservoir (Germany). Sampling intervals of 3–4 days and knowledge of the egg–age distribution made it possible to calculate recruitment of the population for each sampling interval. In every year, high Daphnia densities and high Secchi depth readings (clear-water stage) were recorded in spring. In two of the 5 years, Daphnia abundance remained on a high level throughout summer, whereas in the remaining 3 years a midsummer decline of Daphnia was observed. Birth rates declined while the abundance increased in spring in every year but the timing of this decline in relation to abundance and the onset of the clear-water stage revealed no obvious pattern, distinguishing between years with or without a midsummer decline. However, cumulative recruitment did reveal a clear pattern: in years without a midsummer decline, cumulative recruitment increased slowly, but continually, starting from a low level in early May. By contrast, in years with a midsummer decline, recruitment increased strongly during one week in May, starting from a relatively high density level (>30 ind. l−1). This quick increase was followed by extended periods with low recruitment. Consequently, in these years the population was dominated by a strong peak-cohort of about the same age during the clear-water stage. If all members of this cohort, while approaching their maximum life-span, die off during a short time interval, this may have a pronounced effect on the initiation of a midsummer decline. From an extensive literature search, evidence is provided that the findings of this study represent a demographic mechanism which is of general importance for the initiation of a midsummer decline in temperate waters.

Reproductive potential of Daphnia galeata in relation to food conditions: implications of a changing size-structure of the population

A field study was conducted in the highly eutrophic, biomanipulated Bautzen Reservoir to investigate the influences of food conditions on fecundity of Daphnia galeata during spring and early summer in four consecutive years. By defining a relation between body size and maximum brood size, the realised reproductive potential (RRP) of Daphnia was expressed as percentage of actual egg numbers of the population in relation to calculated maximum egg numbers. This parameter is independent of the size structure of the population. It was found to correlate generally with mean clutch size. However, deviations occurred if size at maturity (SAM) declined. Extremely low SAM values (∼0.8 mm) were recorded in 2 years, corresponding with increasing food concentrations. Low clutch sizes in these situations can be explained by small body sizes of ovigerous females, whereas RRP increased, thus reflecting good food conditions. From the comparison of different reproductive parameters, it is concluded that RRP is the most reliable one with regard to bottom-up influences in case of extremely variable size structure and possible predation impacts. Fecundity of D. galeata was clearly food limited during the clear-water stage. Relatively high POC-threshold concentrations of 0.26 mg C l−1 suggest that food quality was low and may be equally important as food quantity in limiting egg production.

Modeling Daphnia population dynamics and demography under natural conditions

Various approaches to modeling the population dynamics and demography of Daphnia have been published. These methods range from the simple egg-ratio method, to mathematically complex models based on partial differential equations and numerically complex individual-based Daphnia population models. The usefulness of these models in unraveling the population dynamics and demography of Daphnia under natural conditions is discussed. Next to this, an extended version of an existing individual-based Daphnia model is documented (Cladosim) and its application to a typical field data set collected in 1995 in Lake Volkerak is shown. To answer the question which factor was limiting Daphnia numbers during the course of the season food level and temperature in the model were varied and results were compared with those obtained for the observed food level and temperature. These analysis showed that in April temperature was limiting while during May–July and September–October food was limiting. In August neither temperature nor food was limiting. Analysis with a set of size-selective mortality scenarios showed that on average the Daphnia population in Lake Volkerak experienced a mild positive size-selective mortality during the year that was analyzed. Birth rates derived with the detailed individual-based model were compared with those derived with the much simpler egg-ratio method. For the conditions as observed in Lake Volkerak in 1995, both methods gave very comparable results, despite sampling intervals of up to four weeks. The same holds under the environmental scenarios. Using the size-selective mortality scenarios it could be shown, however, that under strong mortality of the smaller daphnids, the egg-ratio method severely underestimates the birth rate. The vices and virtues of the new model and potential extensions are discussed.

The feeding behaviour of large perch Perca fluviatilis (L.) in relation to food availability: a comparative study

The aim of the present study was to analyse and compare the feeding behaviour of large perch (>25 cm total length) in two lakes of different state and corresponding differences in food availability. Investigations were conducted in the Bautzen Reservoir, Germany, (biomanipulated, macrophyte cover <1%) and in Lake Ring, Denmark (non-biomanipulated, macrophyte cover ca. 20%). Fish were sampled regularly using trawl nets and gill nets. We focused our study on seasonal diet patterns. Further, we investigated size-related feeding and predation on both the juvenile fish and invertebrate communities. The availability of Chironominae was high in both lakes (e.g. ca. 1500 ind. m−2 in Lake Ring), but age-0 fish abundance was most probably higher in Bautzen Reservoir (6.4 individuals m−3 in the beginning of June 1998). Invertebrates and age-0 fish were the main food components of large perch of both populations. Mean lengths of prey fish were consistently smaller than those in situ. Large perch in Bautzen Reservoir started earlier to feed on age-0 fish, which was not related to prey fish sizes. Predation on the age-0 fish community was distinctly higher in Bautzen Reservoir than in Lake Ring. Generally, invertebrates represented the most important food of large perch in Lake Ring, whereas the by far dominant food resource of large perch in Bautzen Reservoir was the age-0 fish stock. In conclusion, it can be stated that the prey fish availability was an important factor in determining the feeding behaviour of large perch.

Is the difference in population dynamics of Daphnia galeata in littoral and pelagic areas of a long-term biomanipulated reservoir affected by age-0 fish predation?

We analysed population dynamics of Daphnia galeatain littoral and pelagic areas of the long-term biomanipulated Bautzen reservoir during spring and early summer of two consecutive years. Simultaneously, consumption of the age-0 fish community was estimated by means of a bioenergetics model. As a consequence of biomanipulation, the abundance of adult planktivorous fish in Bautzen reservoir is low and the interactions between underyearling percids and Daphniaare a key process in the food web of this water. In both years, density and mean size of D. galeatawere markedly lower in littoral than in pelagic areas, whereas no differences were found in the size at maturity and clutch size. However, the lower proportion of adults in the littoral zone resulted in drastically reduced birth rates, which could not explain the strong fluctuations in Daphniaabundance. These findings were consistent during the whole investigation period. Daphniaconsumption by age-0 fish was markedly higher in the littoral than in the pelagic zone and accounted for up to 50% and even 100% of Daphniamortality in this habitat. However, due to gape-size limitation, age-0 fish did not feed on daphnids before June. We conclude that besides underyearling percids older planktivorous fish contribute to the suppression of D. galeatain the littoral zone of Bautzen reservoir and that recruitment of daphnids in this habitat mainly takes place via (active or passive) immigration.

Reduction of nutrient loading and biomanipulation as tools in water quality management: Long-term observations on Bautzen Reservoir and Feldberger Haussee (Germany)

Abstract Long-term (1976–1999) biomanipulation in Bautzen Reservoir (BR) revealed that a combination of piscivore stocking and catch restrictions for piscivores led to the desired effects of low planktivorous fish biomass and enhanced biomass of large filter feeders (Daphnia galeata). Despite the hypertrophic status of BR, fisheries management shifted the planktivore-dominated fish community into a piscivore-dominated community. High winter (Jan-Mar) Daphnia biomass was a sensitive indicator of reduced planktivory. Although edible phytoplankton was suppressed by elevated Daphnia biomass, mean seasonal (May-Oct) total phytoplankton biomass remained unchanged due to growth of large inedible algae and cyanobacteria. Inedible and total phytoplankton biomass was primarily controlled by phosphorus availability. However, during clear water periods a reduction of total phytoplankton was achieved with drastically increased Secchi readings. In Feldberger Haussee (FH), despite intensive long-term manual removal of cyprinids (1985–2002) and stocking of piscivorous fish (1988–2002), biomanipulation only had restricted, delayed, or transient effects on the ecosystem. Mean proportion of piscivores within total yield increased but was below 20% in most years. Planktivore cyprinid yields dropped until 1990 and remained constant thereafter. Daphnia biomass slightly increased after biomanipulation became effective (1987–1989) but declined to pre-biomanipulation levels later in the experiment (1996–2005). Beginning in 1997, both edible and inedible phytoplankton biomass started to decrease. Finally, when biomanipulation had been implemented after a delay of 8 years water clarity increased significantly. Although external loading reductions and biomanipulation in both lakes resulted in moderate phytoplankton biomass reduction and Secchi depth enhancement, the reasons for the observed changes were different. Even though strong cascading effects were detected at the top of the food web in BR, the trophic cascade was largely decoupled between phyto- and zooplankton. External plus internal phosphorus loading still exceeded a critical threshold below which a top-down-induced indirect effect of phosphorus sedimentation and finally limitation could have reduced phytoplankton biomass. In constrast to BR, the critical phosphorus loading threshold in FH has probably been approached. Nonetheless, cascading effects were weak due to insufficient reduction of planktivorous cyprinids. Improved water quality was primarily a result of resource-related effects. Thus, the 2 long-term experiments reveal that (1) biomanipulation cannot be applied successfully without reducing nutrient loading below a critical threshold (BR), and (2) for successful biomanipulation, 30–40% piscivores within total fish standing stocks are required (BR and FH). Without optimum piscivory, manual removal of planktivores will hardly produce sustained cascading effects (FH).