Influence of food availability and nutritional state of macroalgae on development of fouling bryozoans on cultivated Saccharina latissima
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- Institutt for biologi 
Fouling of bryozoans is a major challenge in the Norwegian macroalgae industry. Bryozoans are one of the prominent epiphytes colonizing macroalgae, causing mechanical damage, competing for nutrient, shading and decreasing reproductive output. Farming of macroalgae has mainly been undertaken in relatively exposed waters, where nutritional conditions are most optimal and potentially harmful biological interactions less severe. Resource availability, physical conditions and the interactions among organism are different in sheltered and exposed waters and may affect the nutritional state of the macroalgae accordingly. Bryozoans feed on phytoplankton and their abundance might be different in sheltered and exposed locations. This thesis examines how selected external conditions affected the bryozoan fouling on S. latissima cultured at different physical exposures, with a special focus on how nutritional state of the macroalgae affected the development and intensity of fouling, and how phytoplankton food concentrations affected bryozoan coverage. A field experiment was conducted from February to July 2017, where S. latissima were cultivated at 3m and 8m depths at three locations situated along an exposure gradient off the coast of Norway. Lamina were collected throughout the cultivation period for estimates of bryozoan coverage and intracellularly nitrate concentrations. Water samples were taken with regular intervals for inorganic nutrient and chlorophyll a analysis. Physical exposure was found to have no significant (P >0.05) impact on seaweed cultivated at 3m depth. With regards to cultivation depth, the results revealed a depth dependent growth with higher growth at 3m than at 8m depth, at Sheltered location. Intracellular dissolved nitrate content decreased towards the summer, with significantly higher concentrations at the Exposed location (p<0.05). The results revealed a weak positive correlation between external nitrate concentrations and intracellular dissolved nitrate concentrations. Bryozoan colonies settled in mid-May. Although abundant at all cultivation depths, the results revealed a decrease in bryozoan coverage with an increase in exposure at 3m depth, and an increase in coverage with an increase in exposure, at 8m depth. There was a weak relationship between bryozoan coverage and intracellular nitrate concentrations in the macoalgae, with an increase in coverage with a decrease in intracellular nitrate concentrations over the cultivation period. All locations showed good bryozoan food availabilities with no statistical differences. The less exposed locations showed a strong relationship were the variation in bryozoan coverage can be explained by the variation in chlorophyll a food fraction.