Research Article: Regime shifts in coastal lagoons: Evidence from free-living marine nematodes

Date Published: February 24, 2017

Publisher: Public Library of Science

Author(s): Sergio A. Netto, Gustavo Fonseca, Carlo Nike Bianchi.

http://doi.org/10.1371/journal.pone.0172366

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.

Partial Text

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 then 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 increase it [5]. For a given lagoon status, the combination of rate of accretion and sea rise will determine the volumetric capacity of the lagoon, its import/export status, and the resultant evolution [6]. The relative importance of a particular process in a lagoon depends upon the environmental setting in which the lagoon is located and the evolutionary path followed by a lagoon depends upon the magnitude and relative importance of each of the operative processes [5]. The dynamism of these forces promotes both long-term and short-term changes in these ecosystems. In the long-term (months and years), it influences the connectivity with the sea, while in the short-term (tidal cycles), it affects the amount of seawater inflow. According to the present connectivity with the sea, these coastal water bodies can roughly be divided into three major types: open (permanently connected to the sea), the intermittently open/closed (which includes seasonally or non-seasonally closed or those normally closed), and the closed (presently without permanent open bar, the coastal lakes).

Using particular lagoon status over space as replicates of their evolution over time, we observed that open and closed lagoons are mutually exclusive alternative states of equilibrium, and that ICOLLs are an intermediate or transition phase between them. The gradient in the structural connectivity between lagoons and the sea, due to their regime shifts, changes the movement of resources and consumers, and the internal physico-chemical gradients that directly affected the regional species diversity, abundance and trophic status. Whereas the lack of barriers and the fauna movements through the inlets increased similarity between the more connected lagoons, isolation increased variation in the composition of nematode assemblages with species losses and decrease of trophic diversity between closed lagoons.

We conducted an extensive sampling program, using specific lagoon status over space as replicates of their evolution over time, and observed that open and closed lagoons are mutually exclusive alternative states of equilibrium, and ICOLLs are an intermediate or transition phase between them. The gradual regime shift of coastal lagoons, as they lose connectivity with the sea, changes the movement of resources and consumers, and the internal physico-chemical gradients that directly affected regional diversity, abundance and trophic status. Absence of barriers increased the diversity of nematode assemblages and the similarity between the fauna of more connected lagoons. Isolation increased the variation in species composition between lagoons and similarities within lagoons. As local processes within individual lagoons become increasingly more important as they lose connectivity, depending on the local stable state an alternative regional pattern of biodiversity may emerge.

 

Source:

http://doi.org/10.1371/journal.pone.0172366