Abstract
The first few months of life is the most vulnerable period for fish and their optimal hatching time with zooplankton prey is favored by natural selection. Traditionally, however, prey abundance (i.e., zooplankton density) has been considered important, whereas prey nutritional composition has been largely neglected in natural settings. High‐quality zooplankton, rich in both essential amino acids (EAAs) and fatty acids (FAs), are required as starting prey to initiate development and fast juvenile growth. Prey quality is dependent on environmental conditions, and, for example, eutrophication and browning are two major factors defining primary producer community structures that will directly determine the nutritional quality of the basal food sources (algae, bacteria, terrestrial matter) for zooplankton. We experimentally tested how eutrophication and browning affect the growth and survival of juvenile rainbow trout (Oncorhynchus mykiss) by changing the quality of basal resources. We fed the fish on herbivorous zooplankton (Daphnia) grown with foods of different nutritional quality (algae, bacteria, terrestrial matter), and used GC‐MS, stable isotope labeling as well as bulk and compound‐specific stable isotope analyses for detecting the effects of different diets on the nutritional status of fish. The content of EAAs and omega‐3 (ω‐3) polyunsaturated FAs (PUFAs) in basal foods and zooplankton decreased in both eutrophication and browning treatments. The decrease in ω‐3 PUFA and especially docosahexaenoic acid (DHA) was reflected to fish juveniles, but they were able to compensate for low availability of EAAs in their food. Therefore, the reduced growth and survival of the juvenile fish was linked to the low availability of DHA. Fish showed very low ability to convert alpha‐linolenic acid (ALA) to DHA. We conclude that eutrophication and browning decrease the availability of the originally phytoplankton‐derived DHA for zooplankton and juvenile fish, suggesting bottom‐up regulation of food web quality.
Highlights
Eutrophication and browning are major aspect of environmental change in freshwaters with potentially large impacts on ecosystem functioning (Monteith et al, 2007; Scheffer & Carpenter, 2003; Schindler, 2012)
These taxa are nutritionally different to consumers due to their own feeding preferences and the quality of consumed basal food, and both abundance and reproduction of crustacean zooplankton in natural lakes can limit the quality of their diet
The contribution of eicosapentaenoic acid (EPA) was highest in juveniles fed with fish feed than in juveniles fed on Daphnia diets (ANOVA: F3,17 = 37.6, p < .001)
Summary
Eutrophication and browning are major aspect of environmental change in freshwaters with potentially large impacts on ecosystem functioning (Monteith et al, 2007; Scheffer & Carpenter, 2003; Schindler, 2012). Protein content of microalgae is high (>50% of dry weight) Both cyanobacteria and green algae are unable to synthesize essential FAs including EPA and DHA, and thereby lower the availability of these biomolecules in the food web (Galloway & Winder, 2015; Müller-Navarra et al, 2004; Persson, Brett, Vrede, & Ravet, 2007; Taipale et al, 2013; Taipale, et al 2016b). Juvenile fish frequently feed on multiple zooplankton taxa including rotifers, cladocerans (Daphnia), and copepods (Perga, Bec, & Anneville, 2009; Turner, 2004) These taxa are nutritionally different to consumers due to their own feeding preferences and the quality of consumed basal food (phytoplankton, bacteria, and/or terrestrial POM), and both abundance and reproduction of crustacean zooplankton in natural lakes can limit the quality of their diet. We anticipated that joint reduction of EAAs and FAs at the basal trophic level has cascading negative effects causing decreased growth and survival of fish juveniles (H3)
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