ROCKY SHORE MESOCOSMS: INTERACTIONS OF DENSITY, DIVERSITY AND PRODUCTIVITY OF MACROALGAE AT WEMBURY POINT, DEVON

In the past, biodiversity - ecosystem function studies have been limited to looking at diversity and ecosystem function; the interaction of biodiversity with density and productivity is not so well studied. Similar processes creating positive or negative density effects may also be at the base of biodiversity effects on productivity. Monocultures of three species of rocky shore macroalgae, Corallina officinalis, Mastocarpus stellatus and Fucus serratus and polycultures of all three species were incubated in seawater mesocosms on the shore at Wembury Point, Devon, UK. The density by mass was manipulated across these treatments in a replacement design. The oxygen concentration of each mesocosm was measured pre- and post-incubation and the change in oxygen concentration per gram of algae per minute (AO2) was used as a proxy for primary production. High variation in the values of AO2 was encountered due to diel variation in primary production in macroalgae and natural variation in PAR levels across replications. Despite this variation, a negative relationship was found between density and productivity across monocultures and polycultures due to intra- and interspecies competition for 1 light. Polycultures underyielded in comparison with the most productive monocultures, suggesting a negative effect of inter-species competition that reduced the yield of the most productive species in polycultures. No interaction was found between density and diversity despite competition for light reducing productivity at high levels of density and diversity. Partitioning the effects of intraand interspecies competition was not possible in this study due to the replacement design and the use of AO2 as the measure of primary production. This study therefore showed the limitations of using replacement designs, field mesocosms and real-time measures of primary production in short temi studies. Nevertheless, this study did clarify the relationships of density, diversity and an ecosystem function, primary production, in different species and mixtures of rocky shore macroalgae.Faculty of Scienc