Polychaete invader enhances resource utilization in a species-poor system

Ecosystem consequences of biodiversity change are often studied from a species loss perspective, while the effects of invasive species on ecosystem functions are rarely quantified. In this experimental study, we used isotope tracers to measure the incorporation and burial of carbon and nitrogen from a simulated spring phytoplankton bloom by communities of one to four species of deposit-feeding macrofauna found in the species-poor Baltic Sea. The recently invading polychaete Marenzelleriaarctia, which has spread throughout the Baltic Sea, grows more rapidly than the native species Monoporeia affinis, Pontoporeia femorata (both amphipods) and Macoma balthica (a bivalve), resulting in higher biomass increase (biomass production) in treatments including the polychaete. Marenzelleria incorporated and buried bloom material at rates similar to the native species. Multi-species treatments generally had higher isotope incorporation, indicative of utilization of bloom material, than expected from monoculture yields of the respective species. The mechanism behind this observed over-yielding was mainly niche complementarity in utilization of the bloom input, and was more evident in communities including the invader. In contrast, multi-species treatments had generally lower biomass increase than expected. This contrasting pattern suggests that there is little overlap in resource use of freshly deposited bloom material between Marenzelleria and the native species but it is likely that interference competition acts to dampen resulting community biomass. In conclusion, an invasive species can enhance incorporation and burial of organic matter from settled phytoplankton blooms, two processes fundamental for marine productivity

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 model

Comparing grazing on lake seston by Dreissena and Daphnia: lessons for biomanipulation

Biomanipulation measures in lakes, taken to diminish algal blooms, have mainly been restricted to the reduction of zooplanktivorous fish with the aim to stimulate the grazing pressure by native filter feeders such as Daphnia. However, larger filter feeders like the exotic zebra mussel, Dreissena polymorpha, have been suggested as an optional tool because of their high filtering capacity. We compared grazing by two filter feeders, D. polymorpha and Daphnia galeata, offered seston from Lake IJsselmeer, the Netherlands in two consecutive years: 2002 and 2003. The seston in both years was dominated by the colony-forming cyanobacterium Microcystis aeruginosa. The grazing studies were performed under controlled conditions in the laboratory and samples were analyzed on a flow cytometer, making it possible to quantify grazing on different seston components and size fractions, including cyanobacteria, other phytoplankton (green algae, diatoms, etc.), and detritus. No differences in clearance rates, on a per weight basis, were found between the two grazer species. The clearance rate on cyanobacteria (especially <20 µm) was lower in 2003 than in 2002. In 2003, the microcystin concentration of cyanobacteria was higher than in 2002, suggesting that the observed lower clearance rate in 2003 was due to the enhanced toxin content of the cyanobacteria. Zebra mussels, although indiscriminately filtering all seston groups out of the water, positively selected for phytoplankton in their mantle cavity, irrespective of its toxicity, and rejected detritus. Since no differences in clearance rates were found between the two grazer species, we conclude that for biomanipulation purposes of shallow lakes, native species like the daphnids should be preferred over exotic species like zebra mussels. When the seston is dominated by phytoplankton that cannot be filtered out of the water column by Daphnia, however, the use of zebra mussels may be considered. Care should be taken, however, in the choice of the lakes since the mussels may have severe ecological and economic impacts.