Accumulations of the surf diatom Asterionellopsis glacialis in Cassino beach, southern Brazil, and its relationship with environmental factors

Abstract

Cassino Beach (lat. 32°13’S, long. 52°15’W, Southern Brazil) is an exposed, intermediate to dissipative sandy beach, where dense accumulations of the diatom Asterionellopsis glacialis occur in the surf zone. The frequency of these accumulations, their forcing factors and resulting biomass have been investigated along a weekly sampling program from June/1992 to August/1993 at a fixed station. Data on cell densities, chlorophyll a, physical-chemical variables of surf-zone water, beach morphodynamics features and local meteorological conditions were collected. In addition, nearshore sediment and water (just behind the surf-zone) were collected to determine the density of A. glacialis cells, along with some physical-chemical variables of sea water. Results have shown that cell stocks of A. glacialis are resuspended from the nearshore epibenthic habitat by breaking waves and accumulated at the inner surf zone by shoreward water motion. This physically controlled process result in large phytoplankton biomass concentration, which is available to the trophic chains of the beach and surf zone ecosystem.. Mean chlorophyll a concentration(50.2 μg.l-1) was greater than levels found in adjacent neritic and estuarine waters.Dissolved inorganic nutrients concentrations were relatively high during most of the study period (average values: NO3+NO2+NH4=3.41μM; PO4=1.10μM; SiO4=28.5μM). Although accumulations have been more frequent in winter months, the process could not be regarded as seasonal, since its forcing factors can be effective in any season. These diatom accumulations in Cassino Beach provide an important carbon source for the system, which tends to be self-sustained or even an exporter of organic matter.