Working backwards from streambed thermal anomalies: hydrogeologic controls on preferential brook trout spawning habitat in a coastal stream

Abstract

Abstract. Brook trout (Salvelinus fontinalis) spawn in fall, and overwintering egg development can benefit from stable, relatively warm temperatures in groundwater seepage zones. However, eggs also are sensitive to dissolved oxygen concentration, which may be reduced in discharging groundwater. We investigated a 2-km reach of the coastal Quashnet River, Cape Cod, Massachusetts, USA, to relate preferred fish spawning habitat to geology, geomorphology, and groundwater discharge. Thermal reconnaissance methods were used to locate zones of rapid groundwater discharge, which were predominantly found along the center channel of a wider stream valley section. Pore-water chemistry and temporal vertical groundwater flux were measured at a subset of these zones during field campaigns over several seasons. Seepage zones in open valley sub-reaches generally showed suboxic conditions and higher dissolved solutes compared to the underlying glacial outwash aquifer. These discharge zones were cross-referenced with preferred brook trout redds, evaluated during 10 yr of observation, all of which were associated with discrete alcove features in steep cut banks where stream meander bends intersect the glacial valley walls. Seepage in these repeat spawning zones was generally stronger and more variable than open valley sites, with higher dissolved oxygen and reduced solute concentrations. The combined evidence indicates that regional groundwater discharge along the broader valley bottom is predominantly suboxic due to the influence of near-stream organic deposits; trout show no obvious preference for these zones when spawning. However, the meander bends that cut into sandy deposits near the valley walls generate strong, oxic seepage zones that are utilized routinely for redd construction and the overwintering of trout eggs. In similar coastal systems with extensive valley peat deposits, specific use of groundwater discharge points by brook trout may be limited to morphologies such as cut banks where groundwater flowpaths can short circuit buried organic material and remain oxygen rich.