Environmentally driven changes in Baltic salmon oxidative status during marine migration.

The fitness and recruitment of fish stocks can be markedly affected by environmental disturbances including global warming, eutrophication and contamination. Understanding the effects of environmental stressors on salmon physiology during marine residence is of a global concern as marine survival has decreased. We present a unique combination of physiological responses - antioxidant defence and oxidative damage biomarkers, stable isotopes and contaminant exposure biomarkers - measured from adult Atlantic salmon (Salmo salar) collected at the Baltic Sea and studied in relation to environmental variables and fitness estimates. The results demonstrate that feeding populations of salmon display marked temporal and spatial variation in oxidative status. Better oxidative status of salmon was characterized by a higher amount of reduced glutathione (GSH) and decreased lipid peroxidation (LPX), when the weight-at-age of 3-4-year old sprats was higher and contaminant exposure biomarker (EROD) was lower. Summer season conditions, which included cooler sea surface temperature (SST), higher bottom O2 and less cyanobacteria also indicated conditions for better oxidative status. Summer SST was additionally shown to affected glutathione metabolism enzyme activities. Oxidative status was associated with stable isotopes δ13C and δ15N indicating indirect effect of abiotic conditions and lower levels of the food web. Differences in condition factor and growth were associated with oxidative status in one and two sea winter salmon, respectively. Wild salmon survival was higher in years when they had higher GSH and catalase activity and lower LPX. Enhanced glutathione metabolism and increased protein carbonyls were associated with higher occurrence of yolk-sac fry mortality (M74). Our results show that oxidative status can provide information on exposure to complex combinations of environmental conditions and stressors in the wild and provide a link of physiological function to individual and population level fitness effects.

Metabolism and foraging strategies of mid-latitude mesozooplankton during cyanobacterial blooms as revealed by fatty acids, amino acids, and their stable carbon isotopes.

Increasing sea surface temperatures (SST) and blooms of lipid-poor, filamentous cyanobacteria can change mesozooplankton metabolism and foraging strategies in marine systems. Lipid shortage and imbalanced diet may challenge the build-up of energy pools of lipids and proteins, and access to essential fatty acids (FAs) and amino acids (AAs) by copepods. The impact of cyanobacterial blooms on individual energy pools was assessed for key species temperate Temora longicornis and boreal Pseudo-/Paracalanus spp. that dominated field mesozooplankton communities isolated by seasonal stratification in the central Baltic Sea during the hot and the cold summer. We looked at (a) total lipid and protein levels, (b) FA trophic markers and AA composition, and (c) compound-specific stable carbon isotopes (δ13C) in bulk mesozooplankton and in a subset of parameters in particulate organic matter. Despite lipid-poor cyanobacterial blooms, the key species were largely able to cover both energy pools, yet a tendency of lipid reduction was observed in surface animals. Omni- and carnivory feeding modes, FA trophic makers, and δ13C patterns in essential compounds emphasized that cyanobacterial FAs and AAs have been incorporated into mesozooplankton mainly via feeding on mixo- and heterotrophic (dino-) flagellates and detrital complexes during summer. Foraging for essential highly unsaturated FAs from (dino-) flagellates may have caused night migration of Pseudo-/Paracalanus spp. from the deep subhalocline waters into the upper waters. Only in the hot summer (SST>19.0°C) was T. longicornis submerged in the colder subthermocline water (~4°C). Thus, the continuous warming trend and simultaneous feeding can eventually lead to competition on the preferred diet by key copepod species below the thermocline in stratified systems. A comparison of δ13C patterns of essential AAs in surface mesozooplankton across sub-basins of low and high cyanobacterial biomasses revealed the potential of δ13C-AA isoscapes for studies of commercial fish feeding trails across the Baltic Sea food webs.