Zone-specific longshore sampling as a strategy to reduce uncertainties of SGD-driven solute fluxes from high-energy beaches

Abstract

Submarine groundwater discharge (SGD) from intertidal beach systems forms a source of fresh and saline, often nutrient-enriched water to the near-shore across the world. The chemical composition of solute flux via SGD depends on several factors, including solid phase composition, organic matter supply, hydraulic gradients, as well as pore water residence time and origin, which are partly related to the beach topography. High-energy sandy beaches are especially prone to frequent sediment relocation and a variable topography, often characterized by a ridge and runnel structure. Topography may not only change on a temporal scale, but also in the longshore direction, which may alter SGD-driven solute fluxes. However, sampling high-energy beach sites is challenging and often does not allow for high resolution cross- and longshore sampling. We tested the approach of zone-specific pore water sampling, whereby samples were taken at the high water line, in the runnel, on top of the ridge, and close to the low water line of eight cross-shore transects. This was done in order to assess the longshore variability of pore water composition and flux over a 2.5 km wide beach section. In this study we relied on relatively few data points per transect and still found a characteristic cross-shore distribution of pore water parameters, related to redox conditions of each zone. Recharge zones (high water line and ridge) exhibited higher O2 and NOx concentrations and discharge zones (runnel and low water line) exhibited higher nutrient, DOC, Mn, and Fe concentrations. Furthermore, DOC concentrations in pore waters were lower than in coastal seawater, supporting previous assumptions of rapid organic matter turnover. Nevertheless, we observed large concentration ranges among samples assigned to the same zone, which could partly be related to longshore topography differences. The measured pore water concentrations and calculated SGD-driven solute fluxes indicated that sampling of single cross-shore transects may be representative for a beach section when assessing a redox-independent parameter like Si, while it is less representative for parameters affected by more complex biogeochemical processes like P, NH4, Mn, and Fe. For these, we suggest that longshore distributed data coverage combined with higher resolution sampling in discharge zones is a good strategy to reduce the uncertainty associated with estimating SGD-driven solute fluxes in a practicable way.