C3

State of the art and preparatory work

Bulk stable isotope signatures are often used to identify pathways of organic matter through food webs. The δ¹³C signature in consumer tissue reflects dietary sources and their relative contributions when these are isotopically distinct from one another (Fry 2006). δ¹⁵N tends to increase from food source to consumer. Therefore, bulk ¹⁵N measurements have often been used to infer trophic positions (Fry 2006). However, ¹⁵N bulk enrichment factors may be variable (Popp et al. 2007) and may be influenced by the physiological state of the consumer (Adams & Sterner 2000). Moreover, a reliable isotopic baseline is needed. Filter-feeding mussels have been suggested to provide an appropriate isotopic baseline at the primary consumer level (Post 2002). However, our measurements from Lake Constance indicate that δ¹⁵N signatures of Dreissena polymorpha are not stable, but vary considerably with water depth and season (Yohannes et al. 2014). Alternative to bulk measurements, ¹⁵N signatures of single amino acids have been proposed to infer trophic positions of consumers (Popp et al. 2007, McClelland & Montoya 2002). “Source amino acids” like phenylalanine or serine do not undergo major metabolic processes and their δ¹⁵N remains similar to that of the dietary sources, whereas “trophic amino acids” like alanine and glutamic acid undergo metabolic processes and are enriched in ¹⁵N. Thus, trophic position can be defined by the difference in δ¹⁵N between “source” and “trophic” amino acids. Hence, consumer samples alone, without samples taken from food or diet, can be used to delineate trophic positions and also to trace nutrient- and energy sources. Moreover, this technique also works with archived samples. This advanced method is presently being implemented in our stable isotope laboratory.

A recent study reports consistent differences in community structure and life cycle traits of zooplankton species between western and eastern Lake Constance (Seebens et al. 2012). This may either be due to higher primary production in eastern Lake Constance (Bay of Bregenz) because there are the main inflows that supply the lake with nutrients or, alternatively, due to higher allochthonous (terrestrial) inputs of organic matter to this part of the lake, also because of the inflows (Fuentes et al. 2012). In addition, there are differences in productivity between Upper and Lower Lake Constance as well as between different basins of Lower Lake Constance. We hypothesize that productivity differences entail different trophic positions of primary and secondary consumers such as zooplankton and fish. If this assumption is supported, it should be discernible on a spatial scale in isotopic dissimilarities of consumers between lake parts and on a temporal scale in isotopic shifts resulting from the changes in the nutrient status of the lake. The temporal scale can be explored (I) by analyzing archived samples of zooplankton and fish and (II) by retrieving resting eggs, subfossil chironomid and cladoceran remains from dated sediment cores (Perga 2010).

Research program and role within the RTG:

We hypothesize that differences in the trophic state will be mirrored in the stable isotope patterns (δ¹³C, δ¹⁵N and δ³⁴S) of consumers in the pelagic food web that can be determined both in bulk samples, but also in compound specific stable isotope analyses of amino acids. As the latter will provide estimates of consumer and baseline isotopic signatures from consumer samples, we think that its use will provide more accurate and novel insights in changes of food web structure with changes in lake trophic state as studies based on bulk isotopes. It will allow for a Thus, the isotope project interfaces other projects with paleolimnological orientation (A1, A2, A3, B2), projects that are concerned with spatial differences (A4) and projects that deal with food web processes (B1, B2, B3, C2, C4).