Effect of tidal dynamics on a planktonic community in a coastal lagoon of Baja California, Mexico

Abstract

Flfty-two water samples were collected from the surface at the entrance of a coastal lagoon for evaluation of microheterotrophic processes. The study was carried out during a penod of neap and spring tides over 14 consecutive days. Bacteria, microphytoplankton and zooplankton direct counts were made, along with fractionated respiratory activity. Nutrient, C/N ratio, total organic carbon, particulate organic nitrogen, carbon, seston, dissolved oxygen, particulate silica and chlorophyll a determinations were also made. Phytoplankton biomass was low (9.5 11g C I' ) and was exceeded by bacterial biomass (89.4 11g C I-') at both spring and neap tides. Bacterial abundance was independent of tidal height and increased with intense water movement at spring tides. Autotrophic production was limited by light penetration and nutrient availability. Most of the respiratory activity in the water column (> 50 %) occurred in the 5 0.8 1tn1 size fraction. Bacterial standing stocks sustained carbon production in the water column Significant negative correlation between bacteria and nanoplankton suggests a trophic structure dependent on dissolved organic carbon inputs from marsh flora, followed by a trophic chaln composed of bacteria, nanoplankton, microzooplankton and Acartia sp . It is suggested that food chains stemming from dissolved organic carbon instead of phytoplankton may play a major role in the system since tidal dynamics enhanced bacter~al standing stocks during spring and neap tides.