Growth of pioneer beach plants is strongly driven by buried macroalgal wrack, whereas macroinvertebrates affect plant nutrient dynamics

Sandy beach food webs depend heavily upon marine organic input, such as macroalgae, as internal organic matter productivity is low. The fate, however, of this marine organic material (termed wrack) after being deposited onto the beach and its relation to pioneer vegetation, consisting of annual and perennial beach plants, needs to be further elucidated. In particular, the effect of various drivers, such as wrack burial and macro invertebrate presence, on wrack decomposition is largely unknown on sandy beaches. Also, the subsequent effects of decomposition-driven nitrogen and phosphorus availability on beach pioneer plant growth are not yet understood. We performed a mesocosm experiment manipulating Fucus vesiculosus wrack access to the supratidal amphipod Talitrus saltator, and used Cakile maritima and Elytrigia juncea as phytometers to estimate decomposition-driven, wrack-derived nutrient supply. Buried wrack had a strong positive effect (2-3 fold increase) on plant mass, N and P content of C. maritima compared to surface wrack, while effects on E. juncea were largely absent. In addition, macroinvertebrate-facilitated decomposition was important for increasing nutrient availability, but this did not result in an increase in plant growth. We conclude that the burial of wrack by a thin layer of sand is a crucial driver of beach pioneer plant growth, which is most likely due to an increase in moisture availability. This supports the importance of management practices that allow deposited wrack to remain and be buried on the sandy beach for a long period of time, which will have positive effects on beach pioneer plant growth and possibly embryo dune formation.