Phytoplankton primary productivity around submarine groundwater discharge in nearshore coasts

Abstract

Nutrients supplied from submarine groundwater discharge (SGD) are generally thought to enhance primary production in coastal seas. However, there is little evidence for a direct association between SGD and phytoplankton primary productivity. To elucidate the response of in situ primary productivity to SGD, we conducted comparative experiments at 3 coastal sites in Japan with different SGD types (Site A: Obama Bay; Site B: coast along the western foot of Mt. Chokai; Site C: Beppu Bay) during the summers of 2013−2015. At Site A, which is characterized by seepage-type SGD, we found a significant positive relationship between in situ primary productivity and 222 Rn concentration. This was likely driven by nutrient-limited water column conditions. On the other hand, at the volcanic coastal Sites B and C, which are dominated by spring-type SGD, no clear relationships between in situ primary productivity and 222 Rn concentration were found. Although significant relationships between nutrient concentrations and 222 Rn concentration suggest that SGD acts as a primary nutrient source in these regions, the non-trivial influence of light availability complicates those relationships. Furthermore, lower biomass-specific primary productivity around submarine springs at both sites suggests that submarine springs have negative impacts on phytoplankton growth rates around vent sites, possibly due to changes in local environmental conditions. Our study clarified that the mechanism by which SGD affects phytoplankton production differs from one ecosystem to another because of variable hydrogeographical properties, such as the type of groundwater discharge (i.e. spring or seepage). KEY WORDS:In situ primary productivity · Submarine groundwater discharge · 222 Rn OPEN PEN

Submarine Groundwater Discharge: Updates on Its Measurement Techniques, Geophysical Drivers, Magnitudes, and Effects

“…On the other hand, geochemical tracers such as radium and radon are often used for SGD-ecological studies. For example, Sugimoto et al (2017) conducted field experiments at three Japanese coastal sites to elucidate the influence of SGD on in situ primary productivity of phytoplankton. Before the field experiments, they conducted continuous 222 Rn measurement surveys to visualize the spatial variability and gradient of SGD impacts.…”

“…In a coastal water column, phytoplankton community structure was shown to be altered by SGD (Troccoli-Ghinaglia et al, 2010;Blanco et al, 2011;Adolf et al, 2019). More recently, a direct relationship between SGD and in situ phytoplankton primary productivity in nearshore coastal areas in Japan was found (Sugimoto et al, 2017). In Mediterranean coastal lagoons, Andrisoa et al (2019) provided direct evidence for the role of karstic groundwater and porewater fluxes in sustaining primary production.…”

“…The first location where an impact of SGD on fish abundance was directly recorded was in Obama Bay, Japan (Utsunomiya et al, 2017). There, the effect of nutrients transported via SGD on the food-chain was highlighted, because primary production was shown to be higher near SGD sites (Sugimoto et al, 2017). Furthermore, in the tidal flat of Seto Inland Sea, Japan, abundant juveniles of the marbles sole Pseudopleuronectes yokohamae were found near SGD sites with abundant prey organisms (Hata et al, 2016).…”

“…SGD has been shown to influence marine biota across a variety of coastal ecosystems. However, the mechanisms by which SGD affects primary production as well as marine animals differ from one ecosystem to another depending upon the hydrogeographical properties such as type of groundwater discharge (i.e., spring or seepage), location, species present, nutrient content of the groundwater, SGD flux, among other factors (Sugimoto et al, 2017;Shoji and Tominaga, 2018). Additional research is needed for a comprehensive assessment of SGD's impact on marine ecosystems.…”

High-resolution mapping and time-series measurements of 222Rn concentrations and biogeochemical properties related to submarine groundwater discharge along the coast of Obama Bay, a semi-enclosed sea in Japan

“…The symbols RW, tGW, and RSGD indicate a part of each mixing curve in the sea, light intensity was not considered in this study because of a lack of data. However, light intensity was assumed to rarely limit primary production at the surface, as inferred from the limitation factor for light measured at the bottom in the shallow zone of Obama Bay (Sugimoto et al 2017). F N values for the minimum concentrations of nutrients at each observation point were all nearly 0.0, and F N values for the maximum concentrations were around 0.5, suggesting that nutrients were a major factor limiting primary production at all the observation points.…”