Spatial and temporal patterns of salinity and temperature at an intertidal groundwater seep

Abstract

Spatial and temporal patterns at an intertidal groundwater seep at Cape Henlopen, Delaware, were characterized using a combination of pore water salinity and sediment temperature measurements. Pore water salinity maps, both on a small scale (resolution of 0.1 m over a 1.25 m 2 area) and large scale (1–5 m over a 1710 m 2 area) showed reduced pore water salinities to as low as one-sixth seawater strength in a region 0–6 m from the intertidal beach slope break. In this region, there was substantial spatial variability in pore water salinity at all measured scales (0.1–90 m) alongshore. At −10 cm sediment depth, pore water salinity ranged from 6 to 24 in less than 1 m horizontally. To further characterize spatial patterns in discharge, we used novel temperature probes during summer low tides and found temperatures were much lower in a groundwater seep than the nearby sediment, as much as 8–9 °C cooler at −30 cm sediment depth. Measurements over time using temperature loggers showed that despite strong tidal and diel forcing on surficial sediment temperatures, thermal anomalies due to groundwater discharge persisted over several-day sampling periods and were strongest at −20 and −30 cm depth. Over a seasonal time scale, monthly sediment temperatures in or near a seep were cooler in the summer (by 3–4 °C) and warmer in the winter (by 5–6 °C) compared to a nearby non-seep location. In an ecological context, these temperature differences are equivalent to ∼250 km latitudinal shift northward in summer and ∼380 km southward in winter, a change important in relation to recognized biogeographical boundaries. Our results have substantial implications for hydrological measurements made at sandy intertidal sites like Cape Henlopen. Existing methods such as mass-balance and geochemical tracer techniques integrate over the scales of variability we observed, but are unable to resolve small-scale patterns of groundwater discharge that may be biologically important. Seepage meters can capture spatial variability at the meter scale, but results will be subject to high inherent variability unless spatial patterns are considered in sampling design and replication. Intertidal temperature profile measurements for point discharge estimates will be complicated by varying boundary conditions such as the interaction of insolation and tidal exposure. In addition to these methodological implications, the observed spatial and temporal variability suggests a thermal and salinity habitat envelope that must be considered when attempting to interpret biological productivity, faunal abundance and community composition of benthic species living in and around groundwater seeps.