Submarine groundwater discharge as a source of dissolved nutrients to an arid coastal embayment (La Paz, Mexico)

Submarine groundwater discharge (SGD) was investigated in the southeastern portion of La Paz Bay (Baja California, Mexico) using radon (222Rn) as a natural tracer. In the absence of permanent surface flows in this arid region, we hypothesize SGD is a major regional source of dissolved nutrients. Four spatial surveys showed higher radon and nutrient (NH4, NO3, NO2, PO4 and SiO4) values in the winter than summer lagging rainfall by 3–4 months. The surveys revealed two sites (Balandra and Merito) with a stronger radon signal. Intensive time series (12–24 h) measurements at those sites were used to estimate SGD fluxes using a radon mass balance approach. In Balandra, SGD was estimated to be 0.18 m3 m−2 day−1 and significant correlations between radon and nutrients (NO3 and SiO4) were observed. In Merito, SGD rates were estimated to be 0.10 m3 m−2 day−1 and no correlations between nutrients and 222Rn were observed. The difference between the two sites was interpreted to be related to different components dominating SGD (i.e., fresh SGD in Balandra and saline SGD in Merito). The estimated SGD-derived nutrients fluxes were 2–52, 0.04–0.94, 7–164 mmol m2 day−1 for dissolved inorganic nitrogen, phosphate, and silicate, respectively. These fluxes could explain between 5 and 20 % of the regional marine primary productivity values.