Top‐down biological controls on tropical lake productivity

Abstract

Although productivity is primarily controlled by bottom-up factors (nutrient levels), there are secondary effects caused by biological factors such as zooplankton grazing and algal self-shading. The measurement of grazing effects and determination of the contributions of different nutrient pools should elucidate the roles of nutrient levels and biotic effects and also infer the relative importance of classical and microbial food webs. To produce a series of grazing intensity, after initial screening to remove macrozooplankton, lake water was filtered to remove remaining zooplankton. This was used to dilute whole lake water in a filtered : unfiltered ratios from 0:1 to 1:0, which was then incubated with nutrient enrichment. Results show an increase of up to 20% in productivity for surface midlake waters. To investigate grazing by nutrient dilution/deletion, grazing dilutions were again prepared; however, nutrients were added to produce three different conditions,+N +P (all nutrients), –N (nitrogen omitted) and –P (phosphorus omitted). From productivity measurements, the contributions of internal, external and recycled nitrogen and phosphorus nutrient pools were calculated. Sites at Hippo Point (HP) (midlake) and Malewa River inflow (MR) (near the mouth of the River Malewa) show little difference in the contribution of external nutrients, but remaining productivity is mostly fuelled by recycled nutrients at the MR site. Little difference was found between nitrogen and phosphorus-limited conditions.