Allochthonous subsidies drive early recruitment of a subtropical foundation species

Abstract

Terrestrial nutrients flushed to sea via coastal runoff may provide a trophic subsidy to filter‐feeders, which might be particularly relevant in oligotrophic waters. In this study, we investigated how allochthonous subsidies might modulate the settlement dynamics of the mytilid Brachidontes solisianus, a space monopoliser in subtropical Brazilian shores. We used fatty acid trophic markers to identify the dietary subsidies to nearshore waters during wet and dry periods, and integrated them with key ecological drivers (temperature, rainfall) and physiological factors (nutritional status, size at settlement) acting on early life stages. Like other mytilids, for B. solisianus primary settlement can be followed by resuspension and reattachment (secondary settlement). A higher share of primary settlers may signal localized recruitment, while a larger contribution of secondary settlers would indicate increased migration rates. Through redundancy analyses we found that rainfall was correlated with 1) nearshore fatty acid trophic markers of land origin and 2) the frequency of primary settlers during the wet period, when the share of terrestrial inputs was also highest (13% of nearshore suspended matter). Conditions appeared favorable to localized recruitment through trophic settlement triggers, as settlement pulses of pelagic drifters followed heavy rains. Further, lipid analyses of pediveligers revealed that during the dry period these competent larvae selectively incorporated the essential 22:6ω3 docosahexaenoic acid (DHA) into membrane phospholipids at the expense of reserve lipids, suggesting a dietary deficit. Secondary dispersal also prevailed, indicating that the chance for relocation through secondary migrations during dry periods is likely higher. Overall, our results suggest that coastal runoff leads to allochthonous trophic subsidies in the form of organic‐rich plumes which may strongly modulate the settlement rate and dispersal potential of B. solisianus. Subsidies from land to coastal waters may thus be critical for the stability of benthic communities in regions of low marine primary production.