The influence of environmental factors and phytoplankton blooms on cyphonautes larvae abundance and bryozoan colony development at a kelp farm in central Norway

Seaweed cultivation offers a potentially sustainable solution for biomass production. However, in Norway, biomass quality at kelp farms is affected by biofouling, typically from encrusting bryozoans, such as Membranipora membranacea and Electra pilosa. This study investigated the drivers of bryozoan biofouling at a kelp farm in the coast of central Norway in 2022 and 2023. Environmental variables (temperature, salinity, turbidity, light, nutrients and wind), phytoplankton concentrations (chlorophyll a and size structure), and bryozoan (cyphonautes) larval size, abundance and recruitment on the kelp species, Saccharina latissima, were monitored. Phytoplankton biomass and size structure were monitored because cyphonautes are planktotrophic, therefore, phytoplankton was used as a proxy for food availability. Spring phytoplankton blooms (up to ∼ 6 mg chlorophyll a m^-3) followed increased irradiance and reduced mixing, with cyphonautes larvae showing two main abundance peaks - in April (∼200-400 ind m^-3), 1-2 weeks after the onset of the bloom - and in June (∼450 ind m^-3). Larval abundance was associated with low salinity (value ∼ 32), stratified, fresher coastal waters. Membranipora membranacea larvae were generally more abundant and reached larger sizes (up to 0.6 mm in length) during the spring settlement period (late April-June). Larval size, rather than abundance alone, was most closely related to subsequent colony settlement, highlighting the importance of larval maturity for predicting biofouling risk. Colony abundance, size, and areal coverage were higher in earlier-deployed kelp (October 2022 versus January 2023) and increased exponentially from May (<1 %) to late June (up to 11 %). Recruitment peaked during a sharp increase in temperature (1-2 °C in a few days) and was low during a period with high wind speed (up to 15 m s^-1). Our findings demonstrate that food availability, water column stability, rapid temperature increases and cyphonautes size structure are the dominant factors influencing bryozoan biofouling on kelp.