COS 25-2 - Interactive effects of subsidy type and recipient ecosystem productivity on primary consumer-subsidy relationships

Abstract: Distinct ecosystems are connected by spatial flows of materials (i.e., subsidies) across boundaries between systems. Meta-ecosystem theory expands subsidy dynamics by examining how the transfer of material subsidies between ecosystems affects ecosystem processes and food webs. However, the interplay between attributes of ecosystems and the types of allochthonous material they receive remains largely unexplored. The objective of this study is to investigate the different functional roles of macroalgae subsidies in two ecosystems with drastically different in situ productivity: a low-productivity sandy beach and a high-productivity saltmarsh. We hypothesized that similar subsidies in contrasting ecosystems would have distinct consumer communities associated with them, but the similarity of the subsidy-associated and ambient community would depend on the ecosystem’s productivity. Furthermore, high-quality subsidies will attract more consumers compared to a lower quality subsidy, no matter the recipient ecosystem. We conducted field surveys and an experiment to assess the macroinvertebrate consumer use of allochthonous macroalgal subsidies on an adjacent beach and marsh in Maine, USA. We collected macroinvertebrate samples from naturally occurring macroalgae deposits and the ambient ecosystem in both habitats, then compared the macroinvertebrate community structure in each using PERMANOVA. For the manipulative subsidy addition experiment, we installed macroalgae (Ascophyllum nodosum) derived reproductive receptacles (high-quality, high-palatability) and vegetative fronds (low-quality, low-palatability) in leaf litter bags each ecosystem. We used linear models to compare the macroinvertebrate colonization and consumption of each subsidy in both ecosystems.



Our field surveys revealed subsidy-associated communities in each ecosystem were significantly different, despite being collected from deposits of the same type macroalgae. Moreover, the subsidy-associated and ambient macroinvertebrate communities were similar on the beach, but distinct in the marsh, supporting our hypothesis. The field experiment offered mixed support for our second hypotheses, as macroinvertebrate abundance was highest on the low-quality subsidy in the high-productivity marsh. Yet, the high-quality subsidy supported higher consumer abundance on the beach than the marsh. Our results demonstrated that the breakdown and consumer use of subsidies depends not only on the context in which they are deposited, but the consistency and quality of the subsidy itself. Thus, inherent differences between recipient ecosystems and subsidies themselves influence subsidy function across a meta-ecosystem landscape.