In situ Evidence on the Role of Benthic Invertebrates on the Decomposition of Drifting Algal Mats in a Brackish Water Ecosystem

Following climate change and elevated nutrient loads the proliferation of annual filamentous macroalgae results in the accumulation of drifting algal mats in many coastal areas worldwide. The occurrence of the mats may significantly affect the structure and functioning of the benthic communities. We tested the importance of the benthic herbivores Idotea balthica, Gammarus oceanicus and the suspension feeder Mytilus trossulus on the decomposition of the common drift algae Pilayella littoralis and Cladophora glomerata in a brackish water ecosystem of the Baltic Sea. The field experiments were performed in April-May (29 d) and July-August (29 d, last checkpoint 70 d in autumn). The changes in dry weight, total nitrogen and phosphorus content of the macroalgae were examined with an average of 7 d interval. The results showed that algal decomposition was related to temperature and their net photosynthetic rates. The benthic invertebrates significantly affected the algal decomposition. However, the character of the effect depended on the algal and invertebrate species and the physiological state of algae. When algae were photosynthetically active M. trossulus, through the fertilizing effect of biodeposition, promoted the growth of the algae and hence, retarded the algal decomposition. On the other hand, in the presence of herbivores the complete breakdown of the drift algae was reached already within 30-40 d as compared to the control value of 70 d. G. oceanicus had stronger effect on P. littoralis whereas I. balthica contributed mostly to the decomposition of C. glomerata. It is concluded that herbivores significantly accelerate the decomposition of the drift algae whereas suspension feeders postpone the decomposition as far as macroalgae remain photosynthetically active.