DETERMINATION OF PHYTOPLANKTON CROPS BY TOP-DOWN AND BOTTOM-UP MECHANISMS IN A GROUP OF ENGLISH LAKES, THE WEST MIDLAND MERES

Twenty-four lake basins formed in glacial drift ranged in maximum depth from 1.5 to 31 m and were characterized by generally low inorganic N concentrations and very high total P concentrations. The high P values were in some cases related to farm effluent pollution but in many others, despite annual mean values up to 1.46 mg liter-1, appear natural and due to the local mineralogy. For the entire group, there were no significant relationships between mean growth season chlorophyll a concentration and any measured chemical, morphometric, or zooplankton variable. When the group was divided on the basis of maximum depth and presence or absence of thermal stratification in summer into a shallow group (< 3 m) and a deep group (> 3 m), strong inverse correlation was obtained between chlorophyll a and cladoceran abundance in the shallow group and strong direct correlation with inorganic N in the deep group. Grazer control in the shallow group is probably linked with the dominance of submerged macrophytes and the refuges they provide for grazers. Some of the controversy presently surrounding the relative importance of top-down vs. bottom-up control of phytoplankton populations may thus be removed if consideration is given to the morphometry of the lakes.