Algal decay, temperature and body size influencing trophic behaviour of wrack consumers in sandy beaches

Abstract

Drift macroalgae detached from rocky substrate frequently strand on the shoreline, supplying food and habitat to a number of coastal species and sustaining important ecological processes, such as consumption and biogeochemical processing. Large amounts of algal wrack subsidies in open-coast sandy beaches follow the consumption pathway through the activity of upper shore crustaceans. The potential effect of the consumer population on the fate of algal subsidies depends on the rate at which these allochthonous inputs are exploited. Consumption in turn depends on the edibility of the stranded wrack and the environmental conditions that affect both consumers and consumed materials. The degradative process of algal wrack through the consumption pathway was analysed in a set of laboratory and field experiments where the effect of temperature and decomposition level was tested. Pre-weighted fragments of Saccorhiza polyschides and Sargassum muticum were supplied in independent containers harbouring groups of small, medium and large size classes of Talitrus saltator, a common supratidal amphipod widely distributed along the NE Atlantic and Mediterranean sandy beaches. To check the influence of primary environmental variables, such as temperature, assays were performed along the range of expected temperatures in the region, at 5, 10, 15, 20, 25, 30 and 35 °C. The influence of unpredictable natural events that are endured by the algal material once stranded on the beach was evaluated by mimicking macroalgae drying, burying and ageing along different time lapses. Consumer behaviour in the field was tested by adding individuals and algal fragments into experimental containers, as well as in manipulative assays where algal fragments were deployed on the drift line, by night at low tide, following the peak of consumer abundance on the beach. The consumption rate on fresh material depended on temperature and body size. Both low and high temperatures drastically reduced the consumption of algal material, with the feeding rate increasing with decreasing body size. Decomposition of algae enhanced the consumption, with maximum rates obtained when algae decayed in a wet environment. Consumption rose with increasing algal humidity, with deterrent molecules (such as phenolics) playing a minor role once the decomposition evolved with time.