Existence of a deep subtidal stock of drifting Ulva in relation to intertidal algal mat developments

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Blooms of drifting green algae often develop in shallow coastal zones that receive significant nutrient inputs. Each spring and summer, some fifty bays and coves in Brittany (France) are affected in this way. Until now, in this region, only the algae present in the surf zone or stranded ashore, constituting an intertidal stock, have been taken into account. Another stock of algae, which was subtidal and of the same species ( Ulva spp.), was found in the Bay of Douarnenez, one of the ten areas most affected by these algal blooms. This subtidal Ulva stock was located beyond the surf zone, at depths reaching 15 m. It was about the same size as the intertidal stock, viz., a few thousand tons on average. Subtidal Ulva stocks were generally found lying on the sandy bottom in a distribution showing no particular pattern. Biomass ranged from almost zero to 1.547 kg m −2 of fresh and spun-dried algae. However, at depths from 3 to 7 m they were often arranged in strips a few dm wide, due to the swell's effect. The bottom conditions of temperature, salinity, irradiance and dissolved inorganic nitrogen measured during spring and summer are suitable for the growth of Ulva in the subtidal zone. Both intertidal and subtidal drifting Ulva stocks are mobile and capable of exchanging material. In spring, the intertidal stock's inoculum is likely to come from the subtidal. Later in the season, the subtidal stock could be supplied, at least partially, by the intertidal.