Primary Production in the Plankton Community of a Tropical Lake

The primary production of Lake Lanao, Philippines, was studied over a 15—mo period by in situ application of C—14 and oxygen—difference techniques. Supporting data include weather, water chemistry, light penetration, and standing crop of both autotrophs and heterotrophs. A statistical treatment of production estimates precedes the presentation of data. Extensive comparison of the oxygen and C—14 methods indicates that the C—14 method as applied in Lake Lanao measures net primary production. Data from time—course experiments show no evidence of diurnal rhythms in the efficiency of photosynthesis per unit area of lake surface. Heterogeneity studies based on transect data indicate that at low to moderate levels of production, the probability that production at an index station will differ from the average for the lake on a given date by more than 30% is less than .05, while the comparable probability for high levels of production is .35. There is no significant difference between stations in mean primary production for the study period. Vertical profiles of photosynthesis exhibit light inhibition on all but the most overcast days. The threshold for inhibition at the surface is near 133 kerg/cm²°s during calm weather and somewhat lower in windy weather. The mean threshold for inhibition 1 m or more below the surface is lower than at the surface (101 kerg/cm²°s). The lake is exceptionally transparent (mean extinction coefficient, 0.38) considering its high productivity and has a vertical dispersion of production that is similar to temperate oligotrophic lakes. The characteristic is explained in terms of the low amounts of dissolved and suspended matter in the euphotic zone, high production per unit of standing crop, and great amount of mixing in the upper water column. Net primary production average 1.7 gC/m²°day, and gross primary production is 2.6 gC/m²°day. Autotrophs account for 80% of respiration in the euphotic zone. Factors controlling seasonal variation are related to resource supply rather than to temperature or biomass removal. Between 12 and 30% of seasonal variation in production can be accounted for by variations in incident light. Light limitation also occurs due to thickening of the zone of mixing during the circulation period and during storms. Nutrient supply is the dominant controlling factor during stratification. Nutrient depletion is relieved at frequent intervals by changes in the depth of mixing associated with storms. High sustained production on a low nutrient base is explained by rapid transfer of nutrients from the zone of decomposition back to the euphotic zone. General conclusions are drawn concerning the relative importance of seasonal and aperiodic variation in regulating the resource supply of temperate and tropical plankton communities.