Toxicity of Dinophysis spp. in relation to population density and environmental conditions on the Swedish west coast

The aim of this study in the field was to investigate whether there are differences between the outer archipelago (Gullmar Fjord) and a semi-enclosed fjord system (Koljö Fjord) in occurrences of D. acuta and D. acuminata as well as in their content of diarrheic shellfish toxin (DST) per cell. When all data pairs of cell toxicity of D. acuminata and the corresponding number of cells l−1 from the two sites were tested in a regression analysis, a statistically significant negative correlation became evident and was apparent as a straight line on a log–log plot (p < 0.0001). Obviously, there was an overall inverse relationship between the population density of D. acuminata and the toxin content per cell. Plotted on a linear scale, all data-pairs of cell toxicity and cell number made up a parabolic curve. On this curve the data-pairs could be separated into three groups: (i) D. acuminata occurring in numbers of fewer than approximately 100 cells l−1, and with a toxin content per cell above 5 ρg cell−1; (ii) cell numbers between 100 and approximately 250 cells l−1 with a cell toxin content from 5 to 2 ρg cell−1; (iii) when the population became greater than 250 cells l−1, the toxicity, with few exceptions, was less than 2 ρg cell−1. By applying this subdivision, some clear patterns of the distribution of the differently toxic D. acuminata became evident. When comparing the cell toxicity of the two sites, it was obvious that the D. acuminata cells from all depths from the Gullmar Fjord as a mean were significantly more toxic compared to the Koljö Fjord samples. The results have demonstrated that approximately 100 high-toxicity cells in a low-density population at surface may lead to the same accumulation of DST in a mussel as the ingestion of 1500 low-toxicity cells from a high-density pycnocline population.