State of the art
Copepods are important components of planktonic food webs and dominate the crustacean zooplankton biomass especially in oligotrophic lakes (Straile and Geller 1998). In contrast to Daphnia, their population regulation and food web effects are less well known. Mesocosm experiments (Sommer et al. 2001, 2003) and time series analyses (Huber and Gaedke 2006) have emphasized the differences between copepods and Daphnia, suggesting that copepods are important herbivores on large phytoplankton species including mixotrophic species such as Dinobryon or Ceratium, and are able to strongly suppress ciliates with cascading effects on bacteria (Jürgens et al. 1994; Zöllner et al. 2003). But these studies did not differentiate between the effects of different copepod species. Differences in ecosystem effects between three closely related and morphologically similar calanoid copepods have been demonstrated (Matthews et al. 2011). Hence, it is unclear to which extent copepod species identity is functionally important, and whether copepod diversity in a lake is redundant in respect to ecosystem functioning.
Eutrophication in Lake Constance resulted in large changes in the relative importance of the copepods in the crustacean zooplankton community. During the 1920s, calanoid copepods contributed roughly 60 % to total biomass on annual average, whereas from 1979-1995 this contribution was on average less than 20 %. In contrast the average contribution of cyclopoid copepods almost doubled from roughly 10 to 20 % (Straile & Geller 1998). In addition, the species composition changed. Alterations in the seasonal occurrence pattern of Eudiaptomus gracilis with eutrophication were suggested to be due to intraguild predation by Mesocyclops leuckartii (Seebens et al. 2007). Likewise, the extinction of the calanoid copepod Heterocope borealis in the 1950s was supposed to be due to intraguild predation by the invasive copepod Cyclops vicinus (Einsle 1983). With re-oligotrophication, the changes in relative importance were only partially reversed, Cyclops vicinus is still present in the lake (albeit at lower numbers) and Heterocope was not able to re-invade (Straile & Geller 1998). Hence, when comparing the 1950s with the 2000s there is still a considerable difference in crustacean community composition despite similar phosphorous concentrations. Consequently, the ecosystem consequences and population regulation consequences (e.g., via intraguild predation) of the changes in relative species abundance, and of species invasion are not known and need to be investigated in order to understand ecosystem reversibility at the functional level.
Proposed project and role within the RTG
The project will address the role of species identity for the reversibility of ecosystem functioning. If the invasive copepod Cyclops vicinus precludes the re-establishment of Heterocope borealis, despite a return to suitable trophic conditions, this could be called “niche constriction”.
We will perform bottle experiments on a monthly resolution. 2 L bottles will be filled with lake water filtered through a 200 µm net in order to remove larger zooplankton. Large copepod stages and adults of copepod species present at the different time of the year will be separated and added species-specifically to the bottles. In addition to copepods, we will also have a treatment with Daphnia as grazer. Hence, for each experiment we will have 5 different treatments (3 copepod species, Daphnia + control). Experiments will be incubated for three days in situ as preliminary experiments have revealed good results using this method and this incubation time (Rothhaupt & Straile, unpublished). Thereafter, nauplii, rotifers, ciliates and phytoplankton and bacteria will be identified and counted. Experimental results will also be compared to the seasonal dynamics of the respective groups. In cooperation with project A2 the microbial metagenomics will be investigated for selected experiments.
In addition to the bottle experiments, the doctoral researcher will analyse the influence of copepods on rotifers, ciliates and phytoplankton communities in the long-term data set available for Lake Constance. Time series with data on all players are available during in the eutrophic/mesotrophic period (1987-1996) as well as from the oligotrophic period (2006-2012). We will test whether the influence of copepods on prey communities will change with trophic status. We furthermore expect so see an especially strong negative relationship between copepods and those prey species in our data set, which have been shown to be especially vulnerable to copepod predation in our experiments.
The project cooperates with project A2, via performing joint experiments. It will provide important information on food web relationships relevant for project B1. Finally, there are also strong connections with project C3, which analyses isotopic signatures of different zooplankton species.