Limiting nutrient patchiness and its role in phytoplankton ecology

Limiting nutrient patchiness is exammed as a factor affecting the community structure and species succession of natural phytoplankton communities held in ammonia limited continuous culture at a dilution rate of 0.3 day-'. It was found that under a homogeneous distribution of the limiting nutrient members of genus Chaetoceros dominated and when ammonia was added daily (patchy distribution). Skeletonemu dominated. Intermediate patchiness gave rise to an assemblage dominated by both Chae/ocero.s and Skeletonrma. The nutrient uptake ability of each assemblage was determined three weeks after experiment initiation. Each assemblage was best able to optimize uptake of ammonia under its particular patchy nutrient regime. Optimization of a patchy environment took place by an increased maximal uptake rate (I',,,,,)while optimization of a homogeneous environment appeared to take place by increased substrate affinity (i.e., low K,). This study demonstrates that limiting nutrient patchiness can alter the relative abundance of populations within a community based on each population's ability to exploit the limiting resource under a particular degree of patchiness. We also show that coexistence of two populations might be expected due to the patchiness of a single limiting nutrient. The importance of patchiness in relation to other factors which determine community structure is discussed.