Salinity, temperature, and macroinfaunal communities in groundwater seeps
Date
2006
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Publisher
University of Delaware
Abstract
At Cape Henlopen, Delaware, USA, intertidal groundwater seeps occur along the shoreline, and these seeps have been the subject of recent studies. Within this zone of mixing between surface water and groundwater, sharp temperature and salinity gradients are formed but previous studies have not focused on the small-scale heterogeneity in groundwater seeps. In this thesis, spatial heterogeneity in groundwater discharge was studied by generating a map of pore water salinity at -10 cm sediment depth over a 19 x 90 m area of the sandflat that showed low pore water salinity constrained to the first 6 m from shore, sharp salinity gradients changing 10 ppt within a meter, and patchiness along shore on the scale of 5-10 m. Thermistor probes were used to generate three-dimensional thermal maps with 1-m horizontal resolution at depths of -5, -10, -20, and -30 cm, which showed spatial patterns in thermal anomalies similar to the patterns of low pore water salinity the salinity map. Monthly sediment temperatures measured at -20 cm at both seep and non-seep locations during low tide showed that groundwater seeps were up to 4.5 C cooler in the summer and up to 7 C warmer in the winter than nearby sediment outside the seep. Temperature loggers deployed in autumn, winter, and summer verified that these patterns were persistent throughout tidal and diel cycles. To evaluate the interaction of groundwater seeps and benthic macroinfauna, several methods were employed. Hydraulic conductivity was positively correlated with density of Marenzelleria viridis tubes, a common polychaete inhabiting groundwater seeps. Feeding experiments with M. viridis showed a decrease in feeding rate with an increase in temperature from 20 C to 30 C while the feeding rate in other treatments (20 C to 10 C and a control at 20 C) remained constant. Transects of macroinfaunal cores along temperature gradients and salinity gradients showed that M. viridis was abundant at all points in the transects, but overall macroinfaunal diversity was negatively correlated with proximity to a groundwater seep. These results show that temperature and salinity in groundwater seeps vary on the scale of 1-10 m in space and are less variable than non-seep locations over time scales ranging from tidal cycles, days, and years. These results confirm and extend previous work at Cape Henlopen and other groundwater discharge sites showing that groundwater seeps have an impact on the ecology of benthic organisms by characterizing the variability of temperature and salinity in groundwater seeps over both time and space.