Diversity and composition of macroinvertebrate communities in a rare inland salt marsh

Abstract

Inland salt marshes are rare habitats in the Great Lakes region of North America, formed on salt deposits from the Silurian period. These patchy habitats are abiotically stressful for the freshwater invertebrates that live there, and provide an opportunity to study the relationship between stress and diversity. We used morphological and COI metabarcoding data to assess changes in diversity and composition across both space (a transect from the salt seep to an adjacent freshwater area) and time (three sampling seasons). Richness was significantly lower at the seep site with both datatypes, while metabarcoding data additionally showed reduced richness at the freshwater transect end, consistent with a pattern where intermediate levels of stress show higher diversity. We found complementary, rather than redundant, patterns of community composition using the two datatypes: not all taxa were equally sequenced with the metabarcoding protocol. We identified taxa that are abundant at the salt seep of the marsh, including biting midges (Culicoides) and ostracods (Heterocypris). We conclude that (as found in other studies) molecular and morphological work should be used in tandem to identify the biodiversity in this rare habitat. Additionally, salinity may be a driver of community membership in this system, though further ecological research is needed to rule out alternate hypotheses.

Population growth of microcrustaceans in water from habitats with differing salinities

“…They did significantly better in MRSM water, which had a higher salinity than they were used to. However, they are not found in the marsh, at least not consistently: in 3 years of sampling in the MRSM, we did not observe any copepods, either using microscope-based identifications or molecular metabarcoding ( Cahill et al, 2021 ). It is possible that they are not naturally found at the salt marsh because the level of salinity it reaches could be past the livable range of the species.…”

“…The results of our experiments were inconsistent with the field distributions of both the ostracods and copepods (though note that we have not conducted a broad regional survey of the species). When samples were taken at the MRSM in previous studies from our group and analyzed with both morphological and molecular tools ( Cahill et al, 2021 ), Heterocypris sp. A were mostly found at the site of the seep and sites 20 and 40 m away from the seep, but then dropped off with increasing distance.…”

“…Although the marsh covers an area of 6.5 acres, the area strongly impacted by the salt seep is much smaller, approximately 10 m in diameter ( Lincoln et al, 2020 ). Our previous work has shown that salinity can range up to nearly five ppt at the seep itself but declines rapidly from that point and varies through the year ( Cahill et al, 2021 ). The vegetation of the MRSM has been described ( Albert, 2010 ; Eallonardo & Leopold, 2013 ; Lincoln et al, 2020 ), but the invertebrate community remains relatively unknown ( Albert, 2010 ).…”

“…Ostracods have a history of being used as environmental tracers; species of ostracod have shown a change in the carapace shape with environmental factors such as pH, salinity, and temperature ( Ruiz et al, 2013 ). Barcoding of the ostracods from the MRSM with the cytochrome oxidase I (COI) gene yielded a 95% match to Heterocypris salina ( Cahill et al, 2021 ), but it is not clear if the species is in fact H. salina or a different species. In other studies, H. salina had the ability to survive in water with a salinity ranging from five ppt to around 28 ppt ( Ruiz et al, 2013 ); Ganning (1971) found a preference for six ppt in a Baltic population of H. salina .…”

“…Many species of copepods live in salty conditions, but the ones we worked with are accustomed to a freshwater environment. A thorough sampling of the marsh in 2018 did not yield any traces of copepods at the Maple River salt marsh, despite using with both morphological and molecular methods ( Cahill et al, 2021 ). Other species of freshwater copepod have been shown to react adversely when temperature and salinity change in their environment ( e.g., Boeckella hamata ; Hall & Burns, 2001 ).…”