Abstract
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.
Highlights
The recent scenarios of continuous global warming in aquatic eco‐ systems predict a change in the phytoplankton composition that currently is mostly defined by an increase in cyanobacterial blooms (Huisman et al, 2018; Paerl & Huisman, 2008; Paerl & Paul, 2012)
We focused on two Baltic key species T. longicornis and Pseudo‐/Paracalanus spp. that largely dominated the mesozooplankton community during two contrasting summers at the Eastern Gotland Basin (EGB) of the central Baltic Sea
Lipid‐poor, filamentous cyanobacterial blooms, we found that summer mesozooplankton had plasticity only in the lipid levels, but not in proteins
Summary
The recent scenarios of continuous global warming in aquatic eco‐ systems predict a change in the phytoplankton composition that currently is mostly defined by an increase in cyanobacterial blooms (Huisman et al, 2018; Paerl & Huisman, 2008; Paerl & Paul, 2012). Cyanobacterial blooms impact pelagic food webs in multiple ways It is largely unclear how mid‐latitude mesozooplankton sustain or build‐up their energy pools like lipids and proteins, and receive PUFAs or essential amino acids (AAs), during unpalatable, lipid‐poor, filamentous cyanobacte‐ rial blooms in summer. ( Paracalanus spp. that often included in Pseudocalanus group, Schulz et al, 2012) are usually dominant calanoid copepods in bulk mesozooplankton composition (Corkett & McLaren, 1979; Frost, 1989; Hernroth & Ackefors, 1979) Both copepod species differ in their principal source of energy stor‐ ages that are used during reproduction or overwintering periods with low food availability (Peters, Dutz, & Hagen, 2013; Peters, Renz, Van Beusekom, Boersma, & Hagen, 2006). In this study for the first time, we compared the δ13C patterns of essential AAs in mesozooplankton from the EGB with additional δ13C‐AA data in mesozooplankton from other sub‐basins and dis‐ cussed the potential use of δ13C‐AA isoscapes in the Baltic Sea
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