Can different vegetative states in shallow coastal bays of the Baltic Sea be linked to internal nutrient levels and external nutrient load?

Abstract

Two different vegetative states, i.e. one clear water state dominated by benthic macrophytes and one turbid state dominated by phytoplankton, are commonly found in shallow lakes. In this study we investigated vegetative states in shallow bays of the Baltic proper to see if different vegetative states could be defined. We aimed to couple different types of dominating vegetation to the external load of nutrients. We further investigated the external load of nutrients, the water exchange time and the nutrient level in the water and determined if they could be linked to the vegetative state of a shallow bay and used as management tools. Three vegetative states were found in the shallow (less than 3 m deep) and small bays (varying between 0.01 to 0.23 km2 ) studied. These states were correlated both to the external load of phosphorus from the local watershed and to the total phosphorus (TP) concentrations in the water column. The first state (1) had clear water and low chlorophyll a concentrations (2–7 μg l−1), relatively dense cover (20–36%) of rooted macrophytes and low TP concentrations (below 32 μg P l−1). The second state (2) had very dense stands (45–103% cover) of rooted macrophytes covered by (39–68%) of filamentous green algae. In this state the water was clear, with low chlorophyll a (2–7 μg l−1), intermediate TP concentrations in the water (42–45 μg P l–1) and low TN (200–350 μg N l–1) concentrations. In the third state (3), the unvegetated bottom areas were larger and chl a and TP levels in the water column were high, between 16 to 20 μg chla l−1, > 45 μg P l−1, respectively. One bay (bay 5) did not fit into any of these states and had high chl a and nitrogen concentrations (23 μg chla l−1 and 1198 μg N l−1) although the TP concentrations (34 μg P l−1) and the external load of phosphorus were relatively low. This could be due to the long water exchange time of the bay or indicate alternative vegetative states within an interval of nutrient concentration. Based on our studies we propose that the point at which a bay shifts from one state to another depends on the levels of the external nutrient load, the water exchange time and the depth of the bay. We also suggest that the TP concentration in the water is the single best parameter to indicate the state of a bay. However, more data are needed before this conceptual model can be used as a tool in coastal zone management plans for shallow brackish bays.