Carbon and nitrogen budgets in manured fish ponds on Israel's coastal plain

δC data from manure-loaded, polyculture, freshwater ponds located on Israel's coastal plain, in conjuction with information obtained from gut contents of target fish grown in these ponds, show that 50–80% of the fish yield originated from algal-based food webs harvested primarily after the algae had been processed within the detritus of the pond bottom. The ramaining yield originated from manure-based food webs. In manured ponds at Dor, net primary productivity (PP) fixed an average of 5 g carbon m −2 day −1 in algal growth. Manuring added an average of 3 g organic carbon and 0.3 g inorganic nitrogen m −2 day −1. Losses of organic matter inherent in the microbial processing of the detritus prior to consumption by target animals require that, to sustain the measured fish yields, more than half of the natural food production was harvested by the target animals. The nitrogen required to sustain the measured PP in Dor ponds depended upon rapid recycling of algal nitrogen via fish and seston community metabolism, and microbial processing of algae precipitated to the detritus.