Furthering the understanding of material and energy flow from the lower to the higher trophic levels in coastal lagoons has received little attention, despite being beneficial for the management and conservation of these highly productive systems, as well as of the adjacent coastal waters. The use of the isotopic values of primary consumers which link the base of the food web (primary producers) to secondary consumers as ‘end-members’ in the isotope mixing models, is a major advance in stable isotope analysis. This can help improve understanding of the contribution of primary consumers to the diet of secondary consumers or other consumers higher up the food web (Fig. 1). However, that isotope mixing models can be highly sensitive to spatial and temporal variations in the values of their end-members. In addition, even a few ‰ differences in delta-13C values may result in significant differences in the interpretation of the results.
Major spatial variations in the delta-13C values of SOM and deposit feeders occur at short distances from the shore. This is due to interlinked and spatially changing environmental conditions, including water hydrodynamics and sediment characteristics, and biological interactions.
Coastal lagoons are characterised by wide fluctuations in environmental variables that lead to large changes in biological components, including benthic assemblages, at various spatial scales, e.g. from 10 to 1000m12. However, little is known about the amount of this variability in environmental and biological features which is transferred in the isotopic values of primary consumers in coastal lagoons.
Through a multifactorial sampling design, we documented, for the first time, large spatial variations,up to 6‰, in the delta-13C values of sedimentary organic matter (SOM) and deposit feeders (e.g., Gammarus aequicauda and Hydrobia acuta), at short distances (ca. 200m) from the shore, in the Cabras lagoon of the Oristano lagoon-gulf system (Fig. 2). This is due to a nearshore-offshore gradient in relation to a series of interlinked and changing environmental and biological factors, such as water depth, water residence time, current velocity, sediment grain-size composition, organic matter content, and biological interactions. For the filter feeders (Ficopomatus enigmaticus and Amphibalanus amphitrite), these spatial variations do not occur, possibly because of a weaker link with SOM (Fig. 2). Therefore, spatial variations in the delta-13C values of primary consumers, in particular deposit feeders, at short distances from the shore should be taken into account, if they are to be used in identifying the diet of secondary consumers in these highly variable systems.
This study can be beneficial for the management and conservation of these highly productive systems, as well as of the adjacent coastal waters.
Delta-15N values in the Cabras lagoon fall in the upper range or are even higher than those reported in densely populated and industrialised coastal areas of Europe, indicating highly eutrophic conditions of the studied area within the Oristano lagoon-gulf system.
This study provides a comparison of delta-15N values reported in the literature for different components of a food web (e.g. primary producers, seston, deposit and feeders). Our results are in accordance with the hypereutrophic conditions of the Cabras lagoon characterised by annual mean chlorophyll-a concentrations3 of 40µg/L, due to a high input of both organic and inorganic anthropogenic nitrogen from a large catchment area which is extensively exploited for agricultural activities.