The identification of relevant scales of variation is a necessary prerequisite before explanatory models can be proposed and tested. In this study spatial and temporal patterns of biomass distribution were assessed for two Ulva morphologies in two
Irish estuaries heavily affected by green tides (wet biomass>1 kg m−2 during the peak bloom). Moreover, using
genetic markers, the species composition of these green tides was assessed. Results revealed that these blooms
were multi-specific, with Ulva prolifera, U. compressa and U.rigida the most frequent species. The species U.
prolifera and U. compressa usually showed a tubular morphology, while U. rigida was mainly laminar. A seasonal
succession common to both estuaries was also identified, with the bloom dominated by tubular species during
spring and early summer, and co-dominated by tubular and laminar morphologies during late summer and
autumn. Moreover, tubular and laminar morphologies exhibited different distribution patterns, with tubular
morphologies varying at bigger spatial scales and higher biomass than the laminar. As tubular and laminar
morphologies exhibited different distribution patterns, varying tubular morphologies along bigger spatial scales
with higher biomass levels than the laminar. Considering that tubular morphologies were usually anchored to
the sediment, while laminar Ulva were usually observed free-floating, these differences could explain a differential
influence by water motion. An important annual and decadal variability in biomass levels of Ulva was
observed, in the case of the Tolka estuary a noticeable increase over the last two decades. These findings should
be considered for the development of management and monitoring strategies since the different habitat of laminar
and tubular morphologies (anchored vs. free-floating) may play an important role in the balance of nutrients
and biomass in the estuary, or determine the response to pollutant exposure.