Seasonal feeding patterns, growth dynamics and the impact of warming on the grazing effects of invasive freshwater bivalves

In the present study factors affecting the filtration activity and grazing effects of the freshwater bivalves Corbicula fluminea and Dreissena polymorpha were investigated. This was done with a combination of laboratory and field related experimental approaches. Both bivalves represent keystone species, who can have a large impact on planktonic community composition and the flux of matter in ecosystems. Additionally the whole seasonal population dynamics of C. fluminea were investigated to provide a basis for future predictions and modelling approaches. The first part of the present work concentrated on the impact of warming on the grazing rates of C. fluminea and D. polymorpha and their natural prey organisms such as heterotrophic flagellates. Therefore filtration experiments were performed in circulating flow channels to provide a natural current by using untreated water from a large river (River Rhine, NRW, Germany). It was shown that the increase of the prey´s growth rates was much stronger than that of the predator´s grazing rates when temperatures were increased from 19°C to over 25°C. When performing the same experiments using a benthic microbial predator community (biofilms dominated by suspension-feeding ciliates), an increase of the grazing rates relative to the growth rates with temperature could be observed. The data shows, that a predator-prey interaction between the macrofauna (Bivalves) and their unicellular prey community can change significantly under the impact of increasing temperature when consumpiton and growth rates develop differentially. This can lead to netto-decrase of the prey abundance. The data suggests that the predator-prey interaction which is balanced at moderate temperatures can become unbalanced with increasing temperature. In the second part of this work the role of Dreissena polymorpha in mediating effects of high summer temperatures on the dominant components of natural river plankton was investigated. It was shown that both heterotrophic flagellates and algae increase in abundance at temperatures above 20°C because of decreasing grazing activity of D. polymorpha at such temperatures. Bacteria, as the main prey of the heterotrophic flagellates, decreased in abundance with increasing temperature, suggesting a trophic cascade (mussel - flagellates - bacteria) that is altered by the temperature response of the mussel ingestion. The data thus demonstrates that microbial communities controlled by a macrofaunal component can experience substantial changes at high summer temperatures because of differential development of direct and indirect grazing effects with temperature. In the third part of the present work, seasonal and interannual variability of C. flumineas grazing activity was investigated. Strong seasonal variations, such as a 50-fold increase from February to July, were observed. These variations were only poorly linked to temperature, as they could be found at both the ambient field temperature and a constant temperature of 15°C. It was shown that highest grazing activity was found at periods of highest reproduction activity. Additionally it was shown that the grazing activity was very low from March to August 2009 compared to the same period in 2008 after the bivalves experienced a period of two weeks with low winter temperatures close to the lethal temperature of 2°C. Such low temperatures lead to reduced reproduction rates as shown in previous studies. It was demonstrated that other factors besides temperature probably associated to life-history can have a large impact on the grazing activity of bivalves. The last part of this study concentrated on the shell growth of C. fluminea. The shell growth of more than 50 individuals was observed over a period of more than one year. It was confirmed, that shell growth decreases with increasing shell size. The data was used to generate growth curves over a period of several years with the help of a von Bertalanffy growth model. Additionally the model was used to estimate the age at a given length, the maximum age (7.3 years) and the maximum shell length of C. fluminea in the River Rhine. Taken together, the present study demonstrated that temperature increase can alter a predator-prey interaction when grazing rates of the predators and growth rates of the prey develop asynchronously with increasing temperature. It was shown that warming can differentially influence organisms on different trophic levels by trophic cascading (mussels-flagellate-bacteria). Additionally, the long-term (whole season) studies showed that there are other factors besides temperature such as cold winter periods or reproduction activity that can surpass direct temperature effects. The last part of the study provided basic data about population dynamics of C. fluminea. Both the short-term grazing experiments and the long-term studies provide new patterns and mechanics, which are relevant to accurately predict the performance and the effects of invasive bivalves under changing environmental conditions

Modelling the Impact of Climate Change on Phytoplankton Dynamics and the Oxygen Budget of the Elbe River and Estuary (Germany)

Mini-Symposium: Impacts of Climate Chang

Intra- and interannual variability surpasses direct temperature effects on the clearance rates of the invasive clam Corbicula fluminea

We measured the clearance rates of the filter-feeding bivalve Corbicula fluminea over a period of 2 years. Strong seasonal variations, such as a 50-fold increase from February to July, were observed. These variations were only poorly linked to temperature, as they could be found at both the ambient field temperature and a constant temperature of 15A degrees C. Instead, peaks in the filtration activity corresponded to the spawning activity. Additionally, a strong interannual variability with much lower clearance rates in 2009 than in 2008 was identified. The low clearance rates were linked to a preceding period of low winter temperatures close to the lethal temperature of 2A degrees C and the associated reduced reproduction rates. Our findings demonstrate that other factors besides temperature and body mass can strongly affect clearance rates. These results should be considered when predicting the effects of changing temperatures on the ecosystem impact of filter-feeding bivalves