Proposals for macrophyte restoration in eutrophic coastal lagoons

Abstract

Based on the comparison of environmental requirements for Ruppia cirrhosa and Potamogeton pectinatus growth, macrophyte versus phytoplankton biomass and production features, and differences in hydrological and nutrient balances between Tancada lagoon (where macrophytes form dense beds) and Encañizada lagoon (with no macrophytes at all), several proposals for macrophyte restoration are presented. The highest photosynthetic efficiency of R. cirrhosa takes place at high irradiance and it grows over a wide range of salinity. P. pectinatus is better adapted to lower light intensity and salinity than Ruppia. R. cirrhosa transplanted from Tancada to Encañizada was successful in enclosures, where light availability increased ( μ = 0.013 cm⁻¹), but not in open waters where light extinction coefficient was 0.032 cm⁻¹. Phytoplankton biomass (0.11–2.15 g C m⁻²) is much lower than macrophyte biomass (16–200 g C m⁻²⁾ in Tancada lagoon. However, phytoplankton production (165 g C m⁻² yr⁻¹ in Tancada, 480 g C m⁻² yr⁻¹ in Encañizada) is the same order of magnitude as macrophyte production (244–467 g C m⁻² yr⁻¹). Turnover rates are 0.3–0.9 day⁻¹ for phytoplankton and 1.2–2.5 yr⁻¹ for macrophytes. Phytoplankton and inorganic particles are responsible for high turbidity of the water in Encañizada lagoon. Phytoplankton blooms in Encañizada lagoon are supported by high freshwater inflows from rice field drains from May to November. The Q_s (seawater discharge)/Q_F (Freshwater discharge) ratios are, respectively, 0.24 and 0.48, which denotes a higher seawater influence in Tancada than in Encañizada lagoon. Decreasing freshwater inputs to Encañizada lagoon both in May and November thus allowing greater inputs of sea water, is proposed as the most effective way to restore this eutrophic coastal lagoon.

The objective being to reduce nutrient loadings to the lagoon and phytoplankton in order to favour macrophyte re-colonization.