Hydrodynamics as a Major Factor Controlling the Occurrence of Green Macroalgal Blooms in a Eutrophic Estuary: A Case Study on the Influence of Precipitation and River Management

Abstract

Data on the variation of physicochemical parameters, biomass and growth of green macroalgae (mostly Enteromorpha) collected between January 1993 and January 1997 in the Mondego Estuary (western coast of Portugal) was analysed with the aim to identify the factors that control opportunistic macroalgal abundance in the system.The annual biomass of Enteromorpha spp. is strongly dependent on the amount of fresh water that enters the system during winter and spring. In turn, the input of fresh water is regulated by precipitation and by river management practices. The optimization of the rice crops from the upstream valley depends on their water level, which determines the number of days and hours per day during which sluice gates remain open in winter and spring. River flow has significant impacts on salinity, N:P ratios, current velocities and light extinction coefficients within the system. The interaction of all these factors controls macroalgal growth and biomass loss processes.In winters and springs during which sluice gates are often closed due to water deficiency of the rice fields (dry winter and spring or dry winter followed by rainy spring), little fresh water enters the system and consequently, salinity remains high, N:P ratios around 20, light penetration increases, and current velocities fall. These conditions facilitate macroalgal fixation, enhance their growth and spring blooms occur. On the contrary, during winters and springs when fresh water is in excess of rice fields' needs (rainy winters and springs), sluice gates remain open for long periods of time. High input of fresh water to the system causes salinity and light penetration to decrease, while N:P ratios and current velocities increase. These conditions contribute both to reduced Enteromorpha growth and higher loss of macroalgal biomass from the system to the ocean.The present work shows that the inter-annual variation of macroalgal biomass in the Mondego Estuary is controlled by hydrodynamics, which in turn depends on precipitation and on river management, according with the water needs of the upstream rice crop. Academic Press

Opportunistic macroalgae metrics for transitional waters. Testing tools to assess ecological quality status in Portugal

“…As a pattern, although we could observe some inter-annual variations as a function of hydrological conditions (e.g. Martins et al, 2001), Ulva spp. biomass tended to increase from early winter (February/March) up to July, when an algal crash usually occurred.…”

“…At first this result might look contradicting with the results of 1993 and 1995. However, it is well identified in this system that macroalgal blooms may not occur in rainy years, due to long intervals of low salinity, in coupled to strong currents occasioned by discharge from the tributary Pranto River (Martins et al, 1999(Martins et al, , 2001). In reality, the accumulated precipitation (from September to June) in 1994 was higher than in 1993 and 1995 (see Fig.…”

“…Water circulation was, until then, dependent on tidal activity and on the Pranto River freshwater input. This small tributary is artificially controlled by a sluice, located at 3 km from the confluence with the south arm, and regulated according to the irrigation needs in rice fields, located upstream (Flindt et al, 1997;Marques et al, 1993;Martins et al, 2001;Lillebø et al, 2005;Neto et al, in press).…”

“…In fact, we could observe a slow recovery of the area occupied by the macrophytes community and the cessation of green macroalgae (Ulva spp.) blooms (Martins et al, 1999(Martins et al, , 2001Marques et al, 2003;Cardoso et al, 2004;Verdelhos et al, 2005;Lillebø et al, 2007;Neto et al, in press).…”

“…We consider this transitional water system (TW) as highly suitable for testing and discussing such type of tools due to the availability of long-term data sets (approximately two decades) and its well known temporal response to anthropogenic influences (namely nutrient enrichment) (Marques et al, , 2003Martins et al, 2001;Cardoso et al 2004Cardoso et al , 2007Verdelhos et al, 2005;Neto et al, in press). It is reasonable to assume that the tools tested and calibrated might be also applicable to sheltered coastal waters systems (CW) where macroalgal growth and accumulation may frequently occur.…”