Seasonal patterns in the maerl community of shallow European Atlantic beds and their use as a baseline for monitoring studies

A study of three shallow Atlantic maerl beds (3-6 m depth) in NW Spain (Galicia) sampled beds every 1.5 months for a year. At each study site, temporal variation in the associated flora and two parameters, temperature and photosynthetic photon flux density (PPFD), were recorded in situ and means were calculated for each period. The range of mean periodic temperatures was 11.9-17.5 degrees C, increasing in late spring-summer and decreasing to < 12 degrees C in winter. Mean PPFD was 24-160 mu mol photons m(-2) s(-1); the maximum was recorded in spring (up to 571 mu mol photons m(-2) s(-1)). The variation in temperature and PPFD was correlated with seasonal changes in the flora. The composition of the associated flora at Benencia Island (Ria de Arousa) showed a strong correlation with day length while at Tambo Island (Ria de Pontevedra), it was correlated with temperature. A total of 127 species was recorded (in seven functional groups), annual algae being the most abundant group. The highest species richness and total percentage cover occurred in spring and summer, and the lowest in autumn and winter. Significant seasonal differences were detected at each site, particularly between summer and autumn-winter, due to variation in cover of the foliose species Ulva rigida and Dictyota dichotoma. We propose that future monitoring of European Atlantic maerl beds should include several study areas and two sampling seasons (preferably summer and late autumn-winter).