Changes in benthic diatom assemblages in shrimp culture ponds.

 Although diatoms constitute part of the shrimp diet, the species composition and structure ofdiatom assemblages that proliferate in the ponds of shrimp farms had not been studied. Because the ponds are manipulated to maintain constant the phytoplankton characteristics (dominated by diatom), here we contrasted the hypotheses that: 1) a single diatom assemblage would be present during a production cycle in a shrimp farm and 2) that the assemblage would be different from those from the natural habitat. Thus, the objective of this investigation was to determine if time variation of diatom assemblages occurs under typical production conditions characteristic of culture ponds in a shrimp farm. Monthly samples where taken (JuneOctober) from the bottom (microphytobenthos) and surface (net plankton) of the ponds. In the bottom samples 100 diatom species and varieties within 46 genera were identified. The best represented genera were Nitzschia (9 species), Amphora (7) and Navicula (6). Variations in diversity values were observed ranging from H´= 1.5 (June) to H´= 4.3 (September and October). In the surface samples 120 taxa (50genera) were identified. Tychoplanktonic forms were dominant, sharing 70% of the taxa with the bottom assemblage. Two temporal assemblages where distinguished in bottom and surface samples: the first one included the June-July samples which showed high dominance and lowest richness, mainly characterized by Amphora coffeaeformis var. salina and Catacombas gaillioni. The second one, September-October, showed lower numbers of the previous taxa plus the appearance of Navicula cryptocephala var. subsalina and Neodelphineis pelagica. The observed changes in the structure of the summer diatom assemblage to an autumn assemblage, similar to that of natural shallow environments, reject the proposed  hypotheses.