Temporal changes of a food web structure driven by different primary producers in a subtropical eutrophic lagoon

Coastal lagoons are often characterized by eutrophic conditions which are known to impair the structure and functioning of both pelagic and benthic compartments. However, the manner in which eutrophication triggers a series of cascade effects in the whole food web in coastal lagoons has received little attention. Using stable isotope (SI) analyses, we investigated the food web structure in the hypertrophic lagoon of Yundang (Xiamen, China) in two periods of the year characterized by the recurrent alternation of Ulva lactuca and phytoplankton blooms in the cool (March) and warm (September) seasons, respectively. Large temporal fluctuations in the dominance of primary producers (i.e. macroalgae vs. phytoplankton) and, thus, in the available food items, were reflected in major changes in the diet and SI signals of several primary consumers, such as the amphipod Grandidierella japonica, the polychaetes Neanthes japonica and Capitella capitata, and omnivorous fishes (i.e. Mugil cephalus, Oreochromis niloticus, and Sardinella zunasi), while these changes were limited in top carnivorous fishes, such as Lateolabrax japonicas. Furthermore, reduced macrozoobenthic abundance available for omnivores in September was found to force omnivores to switch their feeding habits to those of herbivores. The present study provides evidence that the periodical alternation of macroalgal and phytoplankton blooms throughout the year strongly affect the relations among different trophic levels leading to a cascading effect across the whole food web and to major changes in the lagoon's food web structure. Importantly, our study shows that the lagoon's food web structure under persistent eutrophic conditions can still cope with seasonal changes in primary energy source type from macroalgae to microalgae due to the ability of omnivorous fishes to conduit different food sources up to the highest trophic levels. Thus, this study suggests that in such a highly variable eutrophic system, omnivores play a central role in the lagoon's functioning, and help to sustain the biological resources and the ecosystem services provided by the lagoon.