Seasonal variations of bacterial abundance and biomass and their relation to phytoplankton in the hypertrophic tropical lagoon Cienaga Grande de Santa Marta, Colombia

Abstract

The seasonal development of bacteria was studied in the hypertrophic coastal lagoon Cienaga Grande de Santa Mama (Caribbean coast of Colombia). This large but only 1.5 m deep lagoon is subject to strong seasonal variations of salinity from almost fully marine (April/May) to brackish conditions in October/November. Chlorophyll ranged from 6 to 182 μg L -1 , and gross primary production amounted to 1690 g C m -2 per year. Total bacterial number (TBN) ranged from 6.5 to 90.5 x 10 9 cells L -1 and bacterial biomass (BBM) from 77 to 1542 μg C L -1 , which are among the highest ever reported for natural coastal waters. Neither TBN nor BBM varied significantly with salinity, phytoplankton or seston concentrations. Only the bacterial mean cell volume showed a significant relation to salinity, being highest (0.066 μm 3 ) during the period of increasing and lowest (0.032 μm 3 ) during decreasing salinity. Bacterial protein accounted for 24% (19-26%) and phytoplankton protein for 57% (53-71%) of total seston protein. The ratio (annual mean) of bacterial carbon to phytoplankton carbon was 0.44 (range 0.04-1.43). At low phytoplankton abundance [chlorophyll a (Chl a) 100 μg L -1 , BBM was low compared to phytoplankton biomass (the mean ratio was 0.16). In general, BBM varied less than phytoplankton biomass. Most probably, the missing correlation between bacterial and phytoplankton variables was due to (i) organic material partly derived from allochthonous sources serving as food resource for bacteria and (ii) a strong resuspension of bacteria from the sediment caused by frequent wind-induced mixing of the very shallow lagoon.