Pulse of marine subsidies: the role of surf diatom Asterionellopsis glacialis accumulations in structuring the meiofauna of sandy beaches

Resource subsidies are flows of biologically fixed energy and nutrients from one ecosystem to another. Oceanic sandy beaches are typical examples of systems constantly subjected to subsidies. A distinctive feature of some exposed sandy beaches is the brown discoloration of surf zone waters due to rich accumulations of diatoms housed on the sediments seaward of the breaker zone. Large amounts of the rich diatom accumulations may end up on the sediments of surf and the intertidal zones of sandy beaches. In this study, we described the seasonal frequency and compared the strength the subsidies among different beaches by measuring sediment chlorophyll a and total organic content. We hypothesized that large amount of fresh diatom subsidies to beaches sediment will fuel meiofauna from the surf zone to the upper intertidal and change the structure of these communities. Our results supported that pulsed subsidies by the deposition of A. glacialis represented an important resource for the meiofauna. The meiofauna responded quickly to the increase in concentrations of labile organic matter in the sediments along different tidal levels of the beach. This subsidized input, however, is not an isolated process but linked with the passage of cold fronts and significant physical changes in the surf zone. Subsides did not fuel surf and intertidal meiofauna similarly. At the surf zone, subsidies were mainly used by those forms that cope with increasingly turbulent flows that paralleled the accumulation events. Yet in the intertidal zone, the densities and richness of the most abundant meiofauna organisms, the nematodes, thrived together with other meiofaunal organisms. Due to their frequency over the time and differences on space, A. glacialis patch deposits most certainly are not the main food item for high-energy sandy beach consumers, but accumulations do represent a window of opportunity for meiofauna during the year, particularly in periods of food shortage during winter.