At temperate latitudes where seasonal changing environmental conditions strongly affect the magnitude, duration and species composition of pelagic primary production, macrobenthic organisms living below the photic zone rely on the sedimentation of this organic matter as their primary energy source. The succession from nutritious spring blooms to summer cyanobacteria is assumed to reduce food quality for benthic primary consumers and their fatty acid profiles. In contrast, we find low seasonal variability in fatty acid content of five benthic macroinvertebrates spanning two trophic levels in the Baltic Sea, a system with high seasonal variation in phytoplankton species composition. However, levels of the major FA groups vary greatly between benthic species. The results suggest that benthic macroinvertebrates have evolved FA metabolism adapted to degraded sedimenting material. Moreover, our study shows that species composition of benthic macrofauna rather than seasonal changing conditions affect availability of essential nutrients to higher trophic levels.

Seston, sediment, and benthic animals were sampled at a coastal site in the north-western Baltic Proper (58° 48’ 39.5” N, 17° 36’ 25.1” E). The site was situated at 40 m depth with a bottom water salinity varying from 6 to 7 and temperature from 0.18°C to 6.84°C during our sampling occasions. Sampling was done in July, August, October, November 2018, and March, April 2019, representing summer, autumn and spring bloom conditions. Seston was collected as a vertical integrated water sample from the surface to 20 m depth, pre-filtered through a 100 mm mesh and then filtered onto GF/F filters and frozen at -80°C. Sediment was sampled with a boxcorer, subsampled with plexiglass tubes, after which the upper sediment (ca 9 mm) was sliced off, homogenized and frozen at -80°C. Animals were sampled with a benthic sled. We selected adult stages of common macrofauna species, including the surface and sub-surface feeding amphipods, Monoporeia affinis and Pontoporeia femorata, respectively, the facultative suspension and deposit-feeding clam Macoma balthica, the deposit-feeding polychaete Marenzelleria spp. and the predatory priapulid worm Halicryptus spinulosus (referred hereafter to their genera). Animals were sieved from the sediment, placed in aerated filtered seawater for 24 hours in order to facilitate gut evacuation, keeping Halicryptus separate. Macoma was separated from their shell and all animals were frozen at -80°C. For FA and SI analysis, samples were freeze-dried at -120°C (one replicate for seston and sediment, three for each species) and pulverized. Further details of sample collection, and FA and SI analysis are provided in the Supplementary Information. FAs were grouped into major groups: Saturated FAs (SAFAs), Monounsaturated FAs (MUFAs), Polyunsaturated FAs (PUFAs, including EFAs and their precursors Alpha-linolenic acid, ALA and Linoleic acid, LIN), and Bacterial FAs (BAFAs); a full list of individual FAs analyzed is provided in Table S1. 

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