Dissecting the microbial food web:: structure and function in the absence of autotrophs

The balance between organic matter producers and consumers defines the metabolism of aquatic ecosystems. Therefore, we can discriminate between net autotrophic and net heterotrophic ecosystems on the basis of the metabolic predominance of either group. Net heterotrophic systems need allochthonous inputs of organic matter. As a model of a heterotrophic ecosystem, we analyzed the structure and function of the microbial food web in littoral cave lagoons of Mallorca (Spain), where trophic food webs in the absence of light are sustained by organic matter, which enters the system through seepage. We compared the microplanktonic food web of these lagoons with that of net heterotrophic ecosystems which receive both autochthonous and allochthonous organic matter. The upper part of the water column has a similar structure and function to that of the oligotrophic open sea, but with a lower abundance and biomass of organisms, with an average of 4.7 x 10(5) heterotrophic bacteria (HB) ml(-1) and 162 heterotrophic nanoflagellates (HNF) ml(-1). The deep zones are similar to that of the deep sea, with an average bacterial abundance of 2.9 x 10(5) HB ml(-1) and 48 HNF ml(-1) and bacterial production values as low as 0.03 mug C l(-1) d(-1).