Abstract
Gracilaria coronopifolia and an invasive congener, Gracilaria salicornia, were examined across an SGD gradient in the field and laboratory. Tissue samples of both species were cultured for 16 days along an onshore-offshore SGD gradient at Wailupe, Oahu. G. salicornia tolerated the extremely variable salinity, temperature, and nutrient levels associated with SGD. In marked contrast, half of G. coronopifolia plants suffered tissue loss and even death at SGD-rich locations in the field and in laboratory treatments simulating high SGD flux. Measurements of growth, photosynthesis, and branch development via two novel metrics indicated that the 27‰ simulated-SGD treatment provided optimal conditions for the apparently less tolerant G. coronopifolia in the laboratory. Benthic community analyses revealed that G. salicornia dominated the nearshore reef exposed to SGD compared with the offshore reef, which had a greater diversity of native algae. Ultimately, SGD inputs to coastal environments likely influence benthic community structure and zonation on otherwise oligotrophic reefs.
Highlights
Terrestrial groundwater may discharge directly to the marine environment wherever a coastal aquifer is connected to the sea [1]
Benthic community analyses revealed that G. salicornia dominated the nearshore reef exposed to submarine groundwater discharge (SGD) compared with the offshore reef, which had a greater diversity of native algae
In tropical and sub-tropical regions, the influx of terrestrial groundwater to the marine environment typically results in increased nutrient concentrations and decreased temperature and salinity of nearshore waters compared with ambient oceanic conditions [10,11,12,13,14], and these changes relax the nutrient limitations typical of coral reefs
Summary
Terrestrial groundwater may discharge directly to the marine environment wherever a coastal aquifer is connected to the sea [1]. The quality and quantity of groundwater input to marine environments have been well documented, the effects of SGD on biological processes remain understudied [6]. These effects may be amplified in tropical, oligotrophic environments where primary productivity is typically limited by low nutrient levels in coastal waters [7,8,9]. In tropical and sub-tropical regions, the influx of terrestrial groundwater to the marine environment typically results in increased nutrient concentrations and decreased temperature and salinity of nearshore waters compared with ambient oceanic conditions [10,11,12,13,14], and these changes relax the nutrient limitations typical of coral reefs. Increases in algal growth rate, photosynthetic rate, and tissue nitrogen (N) are often observed as nutrient concentrations increase [15,16,17]
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