The use of nutrient reduction and food-web management to improve water quality in the deep stratifying Wupper Reservoir, Germany

Abstract

Only a combination of nutrient load abatement and food-web management proved efficient for the management of water quality in the deep stratifying Wupper Reservoir. Reduction of nutrient loading, was completed in winter 1992/1993, but resulted only in reduced winter/spring mixing of phosphorus concentrations. Since the capacity of the diatom spring bloom to remove nutrients from the trophogenic layer of this slightly eutrophic waterbody was never exhausted, the surplus of total phosphorus available to support summer algal growth remained unchanged. Thus, nutrient reduction alone did not improve the water quality, as expected. Subsequent replacement of the smaller Daphnia cucullata by the larger Daphnia galeata-hyalina complex that was attributable to successful foodweb management did, however, result in a shift from a turbid to a clear water regime in 1999. Clearly, the zooplankton community, and therefore food-web structure, played an integral role in nutrient recycling and in the repartitioning of the phosphorus pool. As diatom settling and grazing became much more tightly linked with the appearance of the larger-bodied Daphnia galeata-hyalina complex, which exploits lower-level food resources as early as May, daphnids increasingly acted as a sink for phosphorus. This increased export fluxes out of the pelagic zone and leaves a smaller surplus of total phosphorus to support the accumulation of summer algae. Consequently, water transparency and total chlorophyll concentrations in summer improved with food-web restructuring, indicating real oligotrophication of Wupper Reservoir driven by internal feedbacks.

Rehabilitation and management of a moderately deep-stratifying reservoir by the use of nutrient reduction and food-web management

“…Taking settling velocities of about 0.9 m d -1 for the PP \ 30 lm and 100-200 m d -1 for the daphnid/zooplankton pellet express (Bloesch, 2004) into consideration, changed particle concentrations and residence times in the trophogenic layer should give rise to an increased P-downward flux (Mazumder et al, 1988;Elser et al, 2000;Scharf, 2007), as e.g. in Wupper Reservoir (Scharf, 2008a). Strengthened grazing control of the never-settling motile cryptomonads not only reduced the retention time of particulate phosphorus but presumably even reduced P-upward flux due to daily vertical migrations (Salonen et al, 2006), particularly contributing to the formerly noticed summer lake water phosphorus increase in May/June.…”

“…Conversely, for deep lakes, the BEThP hypothesis (Biomanipulation Efficiency Threshold of Phosphorus; Benndorf, 1987) predicts that a certain threshold of both external and internal phosphorus loadings (BEThP) must not be exceeded to induce a decrease in lake phosphorus concentrations by foodweb management, as e.g. in Wupper Reservoir (Scharf, 2008a).…”

“…If food-web management is to be successful, the predicted development of cyprinid dominance, as before refilling, should be prevented in the long term (Persson et al, 1991;Kubecka, 1993;Scharf, 2008b) thus enabling the development of larger-bodied daphnids, e.g. Daphnia galeata, giving rise not only to increased transparency but also to increased Pretention and decreased phosphorus-chlorophyll relationships (Carpenter et al, 1985;Benndorf et al, 2002;Scharf, 2008a). Overall, the results of this whole-lake case study not only will improve our knowledge of the management of weakly stratifying lakes lacking the stabilizing mechanisms of macrophytes (Scheffer, 2001), but also our understanding of the relevance of food-web mediated direct and indirect effects, particularly with respect to phosphorus availability (Benndorf et al, 2002).…”

Effects of pelagic food web structure and nutrient concentration on anaerobic methane metabolism in lake sediments

“…In general, abundant herbivorous zooplankton, primarily daphnids, feed selectively on epilimnetic phytoplankton and sestonic organic carbon (Bloesch and Bürgi 1989;Sarnelle 1999). Consequently, due to intensive grazing and thus sparse phytoplankton biomass, sedimentation rates are expected to be lower compared to those controlled by planktivorous fish (Elser et al 1995, Reinertsen et al 1990), unless inedible phytoplankton taxa start to dominate in the community (Benndorf et al 2002;Larocque et al 1996;Scharf 2008). Moreover, planktivorous fish may foster primary producers due to food digestion and subsequent release of both phosphorus and nitrogen compounds (Vanni and Layne 1997).…”

Diatoms as indicators of ecological change in freshwater reservoirs of South East Queensland

“…1.3) indicating that diatom research in reservoirs has been increasing for the last two decades. These 319 publications cover a wide range of topics on various aspects of diatoms, including studies on silica (Conley et al 2000, Znachor andNedoma 2008), food webs (Scharf 2008), and morphology of individual diatom species (Genkal andKiss 1993, Genkal 2012). There are numerous studies on spatial and/or temporal variability of phytoplankton (Marce et al 2007, Caputo et al 2008, Moreno-Ostos et al 2009, Rychtecky and Znachor 2011, Schneck et al 2011, Menezes et al 2013 as well more general composition of the phytoplankton assemblage (Fonseca andBicudo 2008, Zhou et al 2015).…”