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

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.

Cyanobacteria Blooms in the Baltic Sea: A Review of Models and Facts

“…On the contrary, copepod-dominated zooplankton communities may also facilitate cyanobacteria by praying selectively on the eukaryotic competitors (Ger et al, 2016;Hong et al, 2013). Similar results are found by Eglite et al (2019), who could demonstrate based on fatty acids-, amino acidsand stable carbon isotopes analysis, that mesozooplankton obtained essential fatty acids (FAs) and amino acids (AAs) from cyanobacteria via feeding on mixo-and heterotrophic (dino-)flagellates and detrital complexes. Overall, while copepods play an important grazing role in the ecosystem, they will not be able to control cyanobacteria growth in the Baltic Sea (Sommer et al, 2006).…”

DNA metabarcoding reveals trophic niche diversity of micro and mesozooplankton species

“…Arndt proposed that Keratella, through its feeding, enhances leaking of dissolved matter from the algae [ 4 ], thereby supporting the increased biomass of both bacteria and protozoa. Tracer studies suggest that diazotroph nitrogen is mainly incorporated in the Baltic Sea food web by passive leaking of nitrogen compounds by filamentous cyanobacteria [ 64 , 65 ] that stimulate the production of heterotrophic bacteria and picocyanobacteria [ 33 , 34 , 66 ]. Our study confirms that picocyanobacteria are a key resource for the larger zooplankton species, primarily Temora and Centropages, but also, to some extent, Acartia , Bosmina and Synchaeta ( figure 4 a ), and are as such indirectly supported by filamentous cyanobacteria.…”

The potential of fatty acid isotopes to trace trophic transfer in aquatic food-webs

“…Consequently, in the context of library approaches, group differences in FA δ 13 C seem most suitable to complement other methods such as FA source-tracking [60,61] or AA isotope-based library approaches [65]. Nevertheless, FA isotope-based approaches relying on direct source sampling have great potential to assess trophic interactions [13,66,67]. This is reflected by significant differences in standardized FA δ 13 C we found among specific algae species (e.g.…”