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Inorganic nitrogen has a dominant impact on estuarine eelgrass distribution in the Southern Gulf of St. Lawrence, Canada
Author(s) -
Heuvel Michael R.,
Hitchcock Jesse K.,
Coffin Michael R. S.,
Pater Christina C.,
Courtenay Simon C.
Publication year - 2019
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.1002/lno.11185
Subject(s) - zostera marina , estuary , environmental science , salinity , nitrate , oceanography , bay , seagrass , habitat , ecology , biology , geology
Abstract Eelgrass ( Zostera marina ) coverage and height were evaluated in 16 estuaries in the Southern Gulf of St. Lawrence, Canada, using boat‐based sonar surveys to determine the dominant factors in the decline of eelgrass in the region. Eelgrass coverage was modeled from the sonar surveys and quantified as the percentage of available habitat occupied—with habitat being defined by salinity limits (> 10 PSU) and depth (< 3 m). Estuaries showed a marked variation in eelgrass coverage ranging between 6% and 57% of available habitat, with eelgrass absent in the upper estuary of estuaries with the highest nitrate loading. The Dunk River estuary showed a decline in eelgrass coverage between 1967 and 2014, a period of increasing nitrogen loading. Measures of eelgrass height were not related to coverage endpoints, suggesting that height variables are not suitable endpoints for overall eelgrass health. Analysis of the influence of environmental factors showed that the factor that consistently correlated to eelgrass coverage was nitrate‐N loading while the factor most influencing eelgrass height was light attenuation. A nonlinear logistic loading‐effect model relating eelgrass coverage to nitrate‐N loading indicated that 10%, 25%, 50%, 75%, and 90% decline in eelgrass would be expected to occur in response to estuarine nitrate‐N loads of 1.3 kg ha −1 yr −1 , 8.0 kg ha −1 yr −1 , 50.0 kg ha −1 yr −1 , 312 kg ha −1 yr −1 , and 1947 kg ha −1 yr −1 , respectively. These results suggest that inorganic nitrogen loading is the most significant factor to be addressed by environmental managers when addressing eelgrass declines in estuaries similar to those studied.