DISTRIBUTION AND GROWTH DYNAMICS OF EPHEMERAL MACROALGAE IN SHALLOW BAYS ON THE SWEDISH WEST-COAST

Distribution and growth dynamics of ephemeral macroalgae were investig ated in some shallow (0-1 m) bays on the Swedish west coast during the period 1992 to 1994. Variation in cover and biomass was assessed in n ine bays, and in one of them the seasonal dynamics of these algae was followed intensively over three years. Frequent measurements were take n of algal biomass, degree of cover, in situ growth, variable fluoresc ence and C/N-ratios. Irradiance and water nutrient concentrations were measured concurrently with the growth measurements. Ephemeral macroal gae were dominated by Cladophora and Enteromorpha species and occurred in all sampled bays, except one, covering 10 to 100% of the bottom se diment. Generally, a rapid biomass increase was recorded from mid-May, which peaked after six weeks at 400-600 g dwt . m(-2). Later in the s eason, strong variations in biomass, cover and species composition wer e observed, suggesting that these opportunistic algae form a highly dy namic community. Initial growth rates estimated from biomass samples w ere similar to those recorded from in situ cage experiments, and also agreed with growth rates calculated from a model. For all species stud ied growth rate was within the range 10 to 30 g dwt . m(-2). d(-1), ir respective of method used. Low algal C/N-ratios (mean = 12.7) in 1993 (cold and rainy summer) indicated that growth was not limited by nutri ents, but rather by light. In 1994 (warm and sunny summer), mean C/N-r atios were 20, reflecting the opposite situation. The appearance of th ese opportunistic algae in shallow bays which historically had been wi thout macroalgal communities has changed the characteristics of these areas by altering habitat complexity. This could have important conseq uences for trophic interactions involving many species, thereby alteri ng community structure and function.