Seawater Temperature and Tidal Action as Modulators of Ulva Spp. Micropropagules Density in a Eutrophicated Macrotidal Coastal System

Abstract

Micropropagules are the reproductive and dispersal means of macroalgae, often linked to green macroalgal blooms ineutrophicated coastal areas. In San Antonio Bay (North Patagonia, Argentina), increased nutrients have led to Ulva spp.blooms in spring and early summer, coinciding with high Ulva spp. micropropagule (MPU) density at low tide. This studyaimed to describe the variation in MPU densities throughout a year and in a tidal cycle and their relationship with environmental variables. For this, MPU density, macroalgal biomass, weight of mature and immature thalli, and seawater physicaland chemical variables were determined: (1) with a monthly frequency for a year at low tide, (2) during a tidal cycle at onehour intervals covering the low tide period, and (3) at diferent depths in the water column during daytime and nighttimehigh tides. Maximum MPU density (33,983±9553 cel ml−1) occurred in February, while macroalgal biomass peaked inDecember. MPU density, seawater temperature, salinity, chlorophyll-a, and nutrients increased during low tide but decreasedat high tide, with no evidence of vertical stratifcation. MPU density was positively associated with seawater temperatureduring low tide and throughout the year. We conclude that in an environment where nutrients and light are not limiting, MPUvariation is associated with seawater temperature annually and with tidal action daily. High MPU densities during summerraise chlorophyll and dissolved oxygen levels, while tidal fow dilutes and exports MPU. These results provide insights intothe dynamics of the dispersal phase of an opportunistic and globally distributed green algal genus.