Resource Niches of Co-occurring Invertebrate Species at an Offshore Wind Turbine Indicate a Substantial Degree of Trophic Plasticity

Abstract

Offshore wind farms (OWFs) in the North Sea are proliferating, causing alterations in local ecosystems by adding artificial hard substrates into naturally soft-bottom areas. These substrates are densely colonized by fouling organisms, which may compete for the available resources. While the distribution of some species is restricted to specific parts of the turbine, others occur across depth zones and may therefore face different competitive environments. Here we investigate the trophic niches of seven invertebrate species: three sessile (Diadumene cincta, Metridium senile, and Mytilus edulis), one hemi-sessile (Jassa herdmani) and three mobile species (Ophiothrix fragilis, Necora puber, and Pisidia longicornis) that occur in multiple depth zones. We hypothesized that these species would be trophic generalists, exhibiting trophic plasticity by selecting different resources in different depth zones, to cope with the different competitive environments in which they occur. We analyzed δ 13 C and δ 15 N of these species and their potential resources across depth zones. Our results show that most of these invertebrates are indeed trophic generalists which display substantial trophic plasticity, selecting different resources in different zones. Degree of trophic plasticity was not related to mobility of the species. There are two possible explanations for these dietary changes with depth: either consumers switch diet to avoid competition with other (dominant) species, or they benefit from the consumption of a non-limiting resource. Only Diadumene cincta was a trophic specialist that consumed suspended particulate organic matter (SPOM) independent of its zone of occurrence. Altogether, trophic plasticity appears an important mechanism for the coexistence of invertebrate species along the depth gradient of an offshore wind turbine.

Offshore wind farms and the attraction–production hypothesis: insights from a combination of stomach content and stable isotope analyses

“…The isotopic signatures of the benthic Table 6 Standard ellipse areas corrected for small size samples (SEA C -% 2 ) and overlaps (% 2 ) between the isotopic niches of the different fish species collected at the offshore wind turbine (sculpin) and benthopelagic (pouting and juvenile cod) species indicated a diet based on artificial hard substrate prey items, while that of mackerel and horse mackerel showed pelagic feeding preferences. The d 13 C value of sculpin is characteristic of a benthic diet, which is explained by the exploitation of the crab Pisidia longicornis that consumes sediment organic carbon when it occurs at the scour protection layer (Mavraki et al, 2020a). The d 13 C signatures of pouting and cod reflect the signature of the amphipod Jassa herdmani that mainly feeds on zooplankton when it is found on the turbine foundations (Mavraki et al, 2020a).…”

“…The d 13 C value of sculpin is characteristic of a benthic diet, which is explained by the exploitation of the crab Pisidia longicornis that consumes sediment organic carbon when it occurs at the scour protection layer (Mavraki et al, 2020a). The d 13 C signatures of pouting and cod reflect the signature of the amphipod Jassa herdmani that mainly feeds on zooplankton when it is found on the turbine foundations (Mavraki et al, 2020a). The depleted d 13 C values of the pelagic fish species indicate that they consume prey items that rely their diets on primary producers.…”

“…Discrepancy between SIA and SCA can be a result of the differences in assimilation efficiencies among the prey items (Hertz et al, 2017). This discrepancy could also derive from the similarities in the stable isotope ratios of zooplankton and Jassa herdmani (Mavraki et al, 2020a) resulting in a less reliable resolution of the SIA for these two main food resources and leading to the a posteriori combination of these two prey items in the mixing model. A final explanation could also be found in the feeding behaviour of horse mackerels.…”

“…Overall, the results of this study indicate that there are two main pathways supporting the feeding ecology of the fish species associated with OWFs: the 'pelagic pathway' that was based on the consumption of prey items of pelagic origin, such as zooplankton and SPOM, and the 'benthic pathway' that was based on the local benthic production. Both these pathways could lead to local production, since the main benthic prey item, J. herdmani, is a suspension feeder that mainly consumes zooplankton (Mavraki et al, 2020a). Thus, organic matter of pelagic origin might significantly contribute as a food resource due to the predominance of suspension feeders on the artificial hard substrates.…”

Enabling Renewable Energy While Protecting Wildlife: An Ecological Risk-Based Approach to Wind Energy Development Using Ecosystem-Based Management Values

“…However, there is room for improvement for all the cases in applying scientific principles to the collection and analysis of data, beginning with the design of sampling plans, examination of appropriate sampling intervals to capture changes in life history and seasonal signals, trophic interactions, and effects of external factors such as weather patterns and changing climates. Particularly evident was the paucity of studies to address questions related to the potential effects on ecosystem structure, function, and services, as has been shown in other studies [60,61].…”