Field assessment of the potential for small scale co-cultivation of seaweed and shellfish to regulate nutrients and plankton dynamics

The co-cultivation of seaweed alongside shellfish has the potential to regulate local dissolved nutrient concentrations and consequently affect plankton dynamics. Evidence for this has until now come largely from computational modelling and laboratory studies, rather than field studies. Here we report on weekly/bi-weekly profiling of inorganic nutrient concentrations (nitrate, nitrite, ammonium, phosphate, and silicate) over two years (2019-2020) at three sampling stations across a small-scale (16 ha) kelp and mussel farm in Porthallow Bay, Cornwall, UK. Nutrient concentrations were measured in conjunction with a range of related environmental variables, including water temperature, salinity, clarity, and phyto- and zoo- plankton abundance, biomass and community composition. These environmental data were also supplemented with river discharge data. Our results indicate typical seasonal variations in chemical (nutrient), physical (hydrographic), and biological (plankton) parameters across all three sampling stations and no significant reductions in inorganic nutrient concentrations in the water column downstream from the integrated kelp and shellfish farm. We conclude that the effectiveness of nutrient regulation by integrating seaweed and shellfish aquaculture will depend on local climatic and hydro-geochemical conditions (affecting background nutrient inputs), as well as the design and scale of integrated multi-trophic aquaculture (IMTA) systems.