Phytoplankton, physical and chemical microscale variations in three brackish rock pools

Abstract

Variations at the microscale in phytoplankton distribution with respect to physical and chemical variables in three brackish rock pools of different depth, size and exposure were studied during the growth season. Three hypotheses were made about the microspatial characteristics of the rock pool habitat: (i) there are distinct microscale variations in physical and chemical variables, (ii) the microscale distribution of phytoplankton is related to characteristic physical and/or chemical variables, and (iii) microscale variations are more pronounced in deep rock pools. Variations were studied at a 10 cm scale by close interval siphon sampling. Physical and chemical variations were small irrespective of rock pool. Prevailing weather, as well as the similar round basin shape of the pools contributed to the similarities. Variations in phytoplankton microscale distributions were clear and consistent throughout the study period, and less variable in the deepest pool compared to the shallower ones. The distribution of dominant phytoplankton species correlated with microscale variations in temperature, dissolved oxygen and pH, often during seasonal minima or maxima. The microscale correlations implied that short reaction times of phytoplankton enabled them to exploit, or be influenced by, brief, local variations. The results demonstrated the importance of studies at fine scales to disentangle processes even in shallow weather-influenced ecosystems.