Drought effects on pelagic properties in the shallow and turbid patos lagoon, Brazil

Abstract

The effect of a 7-nao drought (La Nifia 1988) was evaluated on pelagic properties in the large Paros Lagoon (30~176 50~176 Frona December 1987 to December 1988, surface water was sampled along the longitudinal axis of the lagoon for temperature (19-29~ salinily (9-31.4), dissolved inoaganie phosphate (0.02--4.73 pM), nlirate (0.05-66.25 gM), nlirite (0.01-3.54 pM), m, nnaoninm (0.09-33.19 gM), silicate (1.11-359.20 pM), phytoplankton chlorophyll a (chl; 0.4-~tL2 nag na ~), primary production (gross PP 1.72-161.82 nag C na ~ h 1; net PP 0.04-126.19 nag C na ~ h 1), and species composition and abundance (42-4,961 ind nal 1). In the wet season the whole system acted as a river and light availabilily limited phytoplankton growth. During the drought frona February to August monthly freshwater runoff was low mad the inflow of marine water to the southern sector generated spatial vmiabilily of the analyzed properties and five functional areas were recognized. The northernmost Gualba River (1) presented low light availabilily and phytoplankton chl concentration compared to the northern limnetic area (2) (chl mean 13.3 pg 1 1; max 41.2 pg 1 1; gross PP mean 52.6 nag Cna ~ h 1), which acted as a biological filter removing dissolved inoaganic nutrients. Silicate concenlration was slrongly diminished in this area due to diatona uptake (Aulaeoseira granulata, 9,330 cells nal 1). In the northern limnetic and cenlral oligohaline (3) areas, phytoplankton bionaass was conlroned by light but nilrogen also played a limiting role. In the southern area (4) that is under marine influence, low chl concenlration (naean 4.5 pg I 1) mad gross PP (naean 28.1 nag C na ~ h 1 coincided with co-limitation of nitrogen mad light while the channel to the ocean (5) was slrongly light limited. This study denaonslrated that low light mad high silicate input had a buffer effect at Paros Lagoon, hmnpering negative expression of cultural eulrophication. The main effect during the drought period occurred in the northern linmetic region, where low silicate values due to diatona uptake led to higher cyanobacteria abundance, mad enhanced mineralization occurred in the cenlral oligohaline lagoon. Increased rainfall resulted in light limitation and decreasing primary production in the entire freshwater lagoon, and the adjacent coastal region benefited from nulrient enrichment.