Naturally-occurring radon (222Rn) and radium isotopes
(223,224,226,228Ra) were used as tracers to study possible relationships between
submarine groundwater discharge (SGD) and microalgal population dynamics
in Little Lagoon, Alabama (USA). We evaluated SGD inputs based on a mass
balance of “excess 226Ra,” the amount above that contributed from Gulf of
Mexico (GOM) waters. The study area was divided into east, west and central
areas and had estimated SGD rates of 1.56, 0.91 and 0.61 m3 s–1, respectively.
Groundwater nutrient concentrations were extremely high with total N
concentrations up to 945 μM and N/P molar ratios up to ~500. Since there are
no significant surface water inputs to the lagoon, SGD must dominate the
supply of new nutrients. The ratio of the phytoplankton pigments zeaxanthin to
fucoxanthin (a proxy of the abundance of cyanobacteria relative to diatoms)
measured in the water column was highly correlated (r = –0.84) with 224Ra/
223Ra activity ratios, a proxy for water ‘age’. Because the source of these shortlived
isotopes is groundwater, this suggests that SGD is a significant driver of
phytoplankton community composition in Little Lagoon.
Naturally-occurring radon (222Rn) and radium isotopes(223,224,226,228Ra) were used as tracers to study possible relationships betweensubmarine groundwater discharge (SGD) and microalgal population dynamicsin Little Lagoon, Alabama (USA). We evaluated SGD inputs based on a massbalance of “excess 226Ra,” the amount above that contributed from Gulf ofMexico (GOM) waters. The study area was divided into east, west and centralareas and had estimated SGD rates of 1.56, 0.91 and 0.61 m3 s–1, respectively.Groundwater nutrient concentrations were extremely high with total Nconcentrations up to 945 μM and N/P molar ratios up to ~500. Since there areno significant surface water inputs to the lagoon, SGD must dominate thesupply of new nutrients. The ratio of the phytoplankton pigments zeaxanthin tofucoxanthin (a proxy of the abundance of cyanobacteria relative to diatoms)measured in the water column was highly correlated (r = –0.84) with 224Ra/223Ra activity ratios, a proxy for water ‘age’. Because the source of these shortlivedisotopes is groundwater, this suggests that SGD is a significant driver ofphytoplankton community composition in Little Lagoon.
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