Abstract
We test the validity of using the regime shift theory to account for differences in environmental state of coastal lagoons as a response to variation in connectivity with the sea, using free-living nematodes as a surrogate. The study is based on sediment samples from the inner and outer portions of 15 coastal lagoons (5 open to the sea, 5 intermittently open/closed, and 5 permanently closed lakes) along the southern coast of Brazil. Environmental data suggested that there are two contrasting environmental conditions, with coastal lakes being significantly different from open and intermittent lagoons. Marine nematode assemblages corroborate these two mutually exclusive alternative stable states (open vs. closed systems), but assemblages from the intermittently open/closed lagoons showed a gradual change in species composition between both systems independently of the environmental conditions. The gradient in the structural connectivity among lagoons and the sea, due to their regime shifts, changes the movement of resources and consumers and the internal physico-chemical gradients, directly affecting regional species diversity. Whereas openness to the sea increased similarity in nematode assemblage composition among connected lagoons, isolation increased dissimilarity among closed lagoons. Our results from a large-scale sampling program indicated that as lagoons lose connectivity with the sea, shifting the environmental state, local processes within individual intermittently open/closed lagoons and particularly within coastal lakes become increasingly more important in structuring these communities. The main implication of these findings is that depending on the local stable state we may end up with alternative regional patterns of biodiversity.
Highlights
Coastal lagoons are transitional aquatic systems that mediate transfers between the terrestrial environment and the ocean, including potential environmental stressors [1,2]
Most of the nematode genera recorded in open lagoons (70.5%) were those typically found in brackish or marine waters. This proportion was reduced in Intermittently open/closed lagoons and lakes (ICOLL) (49%) with an increasing number of brackish/freshwater or freshwater genera
Freshwater or brackish/freshwater genera accounted for 95% of the collected fauna
Summary
Coastal lagoons are transitional aquatic systems that mediate transfers between the terrestrial environment and the ocean, including potential environmental stressors [1,2]. Lagoons are an evolving coastal landform that may go through a cycle from an open embayment, to a partially back-barrier lagoon with progressive infilling, to a segmentation into small lagoons with unstable inlets and lakes [3,4] (Fig 1A). The evolution of coastal lagoons is the result of the balance between the processes which act to reduce the size of a lagoon and those, which act to PLOS ONE | DOI:10.1371/journal.pone.0172366. The evolution of coastal lagoons is the result of the balance between the processes which act to reduce the size of a lagoon and those, which act to PLOS ONE | DOI:10.1371/journal.pone.0172366 February 24, 2017
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