Background information and definitions
Coastal lagoons (areas of shallow, coastal salt water, wholly or partially separated from the sea by sandbanks, shingle or, less frequently, rocks) are highly biodiverse systems. They can also be connected to land water through rivers, and as such host both marine, brackish, and freshwater invertebrate species. However, coastal lagoons are highly threatened by eutrophication, pollution, urbanization, and diverse forms of modification in their watersheds, caused by high levels of human activity in the coastal zones of all continents (Esteves et al. 2008), resulting in biodiversity loss. Coastal lagoons could be restored through various means, including re-salination, hydrological modifications and algae harvesting (Ghosh et al. 2006; Lenzi et al. 2003). Restoring coastal lagoons can potentially help promote local biodiversity and recover lost or declining species of subtidal benthic invertebrates (Mohapatra et al. 2007).
Esteves F.A., Caliman A., Santangelo J.M., Guariento R.D., Farjalla V.F. & Bozelli R.L. (2008) Neotropical coastal lagoons: an appraisal of their biodiversity, functioning, threats and conservation management. Brazilian Journal of Biology, 68, 967–981.
Ghosh A.K., Pattnaik A.K. & Ballatore T.J. (2006) Chilika Lagoon: Restoring ecological balance and livelihoods through re‐salinization. Lakes & Reservoirs: Research & Management, 11, 239–255.
Lenzi M., Palmieri R. & Porrello S. (2003) Restoration of the eutrophic Orbetello lagoon (Tyrrhenian Sea, Italy): water quality management. Marine Pollution Bulletin, 46, 1540–1548.
Mohapatra A., Mohanty R.K., Mohanty S.K., Bhatta K.S. & Das N.R. (2007) Fisheries enhancement and biodiversity assessment of fish, prawn and mud crab in Chilika lagoon through hydrological intervention. Wetlands Ecology and Management, 15, 229–251.
Supporting evidence from individual studies
A before-and-after study in 1996–2004 in a degraded lagoon connected to the Bay of Bengal, east coast of India (Mohapatra et al. 2007) found that, four years after restoring its hydrology, crustacean species richness decreased, but abundance of commercially valued crustaceans increased. There were reductions in the number of prawn species (before: 24; after: 18 but four were new) and crab species (before: 28; after: 14 but seven were new), and an increase in lobster species (before: 0; after: 2). Abundance (as four-year averages of commercial landings) increased by 1,200% for prawns (before: 187; after: 2,430 t), and 1,135% for crabs (before: 10; after: 130 t). No commercial landings were reported for lobsters. Authors report that increases in landings were correlated with increases in salinity after restoration. The ecological status of the Chilika lagoon declined throughout the 20th century. In 2000, channels were dredged or extended to increase connections to the sea and rivers and improve the hydrology. Data were obtained from Orissa state Department of Fisheries for 1996–2000 (pre-restoration), and by the authors for 2000–2004 (post-restoration), following the same sampling methods. Thirty-four landings centres were visited monthly and prawn and crab catches, including the commercially valued species Penaeus monodon, Penaeus indicus, Metapenaeus monoceros, Metapenaeus dobsoni, Macrobrachium sp. and Scylla sp., were recorded (see study for details).Study and other actions tested
A study in 2005 in a lagoon connected to Cape Cod Bay, Massachusetts, USA (Thelen & Thiet 2009 – same experimental set-up as Thiet et al. 2014) found that, three years after restoring its connection to the sea, molluscs had recolonised the lagoon, but molluscan abundance and species richness significantly varied within the lagoon, increasing with salinity and proximity to its connection to the sea. Sixteen molluscan species were recorded across the lagoon. Species richness in Moon Pond (13 species; highest salinity; closest to the sea) was significantly higher than in the central lagoon (9 species; intermediate salinity and distance to the sea) and the northwest cove (2 species, lowest salinity and furthest from the sea). Total mollusc abundance varied spatially within the lagoon, from 0.3 to 3,470 individuals/m2. Abundance of four selected species (softshell clam Mya arenaria; northern quahog Mercenaria mercenaria; blue mussel Mytilus edulis; periwinkle Littorina sp.) followed the same spatial pattern as species richness (see paper for details). In 2002, tidal flow was partially restored to East Harbor lagoon (dominated by freshwater) by opening a culvert connecting to Cape Cod. Previously, no molluscan species were reported. In summer 2005, locations within three areas of the lagoon were surveyed twice (Moon Pond: 10 locations; central lagoon: 30 locations; northwest cove: 10 locations) using cores (0.79 m2) and quadrats (0.45 m2). Molluscs (>2 mm in cores; >0.64 cm in quadrats) were identified and counted. Salinity was measured at each location.Study and other actions tested
A study in 2005–2011 in a lagoon connected to Cape Cod Bay, Massachusetts, USA (Thiet et al. 2014 – same experimental set-up as Thelen & Thiet 2009) found that, between three and nine years after restoring its connection to the sea, species richness and abundances of molluscs that had recolonised the lagoon following reconnection were decreasing over time, but effects varied geographically within the lagoon. Species richness decreased from 16 in 2005 to eight in 2011 across the lagoon, due to significant decreases in Moon Pond (from 14 to 5). Abundance of mollusc species also declined over time (total values not provided, see paper for details on each species abundance). Abundance of the softshell clam Mya arenaria (the dominant species in the lagoon and only one present in all areas each year), declined between 2005 and 2011, from 3,200/m2 to 8/m2 in Moon Pond and from 2,900/m2 to 7/m2 in the central lagoon, and remained low in northwest cove (0.2/m2 in 2005, 1/m2 in 2011). In 2002, tidal flow was partially restored to East Harbor lagoon (dominated by freshwater) by opening a culvert connecting to Cape Cod. Previously, no molluscan species were reported. In summer 2007, 2008 and 2011, locations within three areas of the lagoon were surveyed (Moon Pond: 20 locations; central lagoon: 24–30 locations; northwest cove: 4–15 locations) using cores (0.79 m2). Molluscs (>1 mm) were identified and counted. Data were compared to 2005 data from a previous study by Thelen & Thiel (2009) summarised above.Study and other actions tested