Morphometry, Primary productivity and Energy flow in a tropical Pond

Abstract

Monthly changes in the morphometric features of pond Idumban reveal that total and littoral areas progressively decreased from 62.4 and 15.4 ha in October-November-December to 6.8 and 2.6 ha in September. The dominant macrophytic producers in the littoral area of the pond were Chara fragilis, Hydrilla verticillata and Ceratophyllum demersum, which flourished from October for a period of 8 to 10 months, exhibiting a typical exponential or 'J'-shaped population growth curve. Biomass of Ch. fragilis increased to the maximum of about 420 g dry weight/m² in April, H. verticillata to 260 g/m² in June-July and that of Ce. demersum to 140 g/m² in April-May. In terms of unit weight, H. verticillata proved to be the most efficient, producing 156 mg dry substance/g plant/day; however, the mean values obtained for 1973-74 were 93, 54 and 53 mg/g/day for H. verticillata, Ch. fragilis and Ce. demersum; the corresponding NPP values 50, 29 and 30 mg/g/day. The GPP and NPP values, expressed in g dry weight/m²/day, were 7 and 4 for H. verticillata, 8 and 4 for Ch. fragilis, 3 and 2 for Ce. demersum. These values, expressed in mg C/m²/day, averaged to 8.2 and 4.6 for all the macrophytes and suggest that the macrophytes were photosynthetically faster and more efficient than phytoplankton. Total gross productivity for Idumban pond amounts to 1773 and 1449 ton (dry weight) for 1973 and 74, respectively: the corresponding values for the NPP were 953 and 825 ton. In other words, 44% of the GPP is lost on plant respiration. Plants equivalent to 56 ton (6% of NPP) are exposed to death in the periphery of the littoral area. The population of Pila globosa proved to be the dominant consumer of these macrophytes. Mean predation amounted to 64 ton/annum for Pila and 200 ton/ annum for other consumer animals. Solar energy known to enter the pond is 1,956,000 Kcal/m²/year. Of this, 24,682 Kcal/m²/year was fixed by the macrophytes, i.e. the photosynthetic efficiency is 1.3%. Of the total GPP, NPP amounted to 13,696 Kcal/m²/year; the net primary production efficiency is 56%. Consumption of the macrophytes by Pila population amounted to 2,943 Kcal/m year and the exploitation efficiency is only 21%.