Hydrogeochemical controls on brook trout spawning habitats 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 are also sensitive to dissolved oxygen concentration, which may be reduced in discharging groundwater (i.e., seepage). We investigated a 2 km reach of the coastal Quashnet River in Cape Cod, Massachusetts, USA, to relate preferred fish spawning habitats to geology, geomorphology, and discharging groundwater geochemistry. Thermal reconnaissance methods were used to locate zones of rapid groundwater discharge, which were predominantly found along the central 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 and evaluated during 10 years of observation, all of which were associated with discrete alcove features in steep cutbanks, where stream meander bends intersect the glacial valley walls. Seepage in these repeat spawning zones was generally stronger and more variable than in 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. Stable water isotopic data support the conclusion that repeat spawning zones are located directly on preferential discharges of more localized groundwater. In similar coastal systems with extensive valley peat deposits, the specific use of groundwater-discharge points by brook trout may be limited to morphologies such as cutbanks, where groundwater flow paths do not encounter substantial buried organic material and remain oxygen-rich.

Hillslope groundwater discharges provide localized stream ecosystem buffers from regional per‐ and polyfluoroalkyl substances contamination

“…The stream is one of the largest on western Cape Cod, and strong coupling to groundwater is evident in observed proportional changes in local groundwater‐level elevation and streamflow (LeBlanc, Guswa, Frimpter, & Londquist, 1986; Walter et al, 2018). Previous work has shown that groundwater discharge to the lower section of the stream is highly preferential in nature, with metre‐scale streambed seepage zones focusing groundwater flux to the river, the dynamics of which are driven by a combination of discontinuous peat lenses in the streambed and heterogeneous streambank sediments (Briggs, Harvey, et al, 2018; Rosenberry et al, 2016). Paired air/stream‐water annual temperature measurements indicate the groundwater flowpaths that supply baseflow to the stream are predominantly deeper in origin (i.e., from greater than approximately 6 m depth below land surface, Briggs, Johnson, et al, 2018).…”

“…Paired air/stream‐water annual temperature measurements indicate the groundwater flowpaths that supply baseflow to the stream are predominantly deeper in origin (i.e., from greater than approximately 6 m depth below land surface, Briggs, Johnson, et al, 2018). However, there is an apparent mixture of local‐hillslope and regional groundwater sources as determined on the basis of specific conductance (SpC), dissolved oxygen (DO), and isotopic sampling (indicating influence of evaporation) of discharging groundwater along the lower 2 km of the stream (Briggs, Harvey, et al, 2018). The Quashnet River valley is broad in some regions, and narrow with the stream in close proximity to steep, sandy hillslopes in others, as shown by the map of land‐surface slope derived from light detection and ranging (lidar) data (https://docs.digital.mass.gov/dataset/massgis-data-lidar-terrain-data; Figure 1).…”

“…Samples of the discharging groundwater were collected at a subset of the spatially distributed observed bank and streambed discharges, including three locations of known repeat brook trout spawning (Briggs, Harvey, et al, 2018), on August 14–18, 2017. Additional surface‐water samples were collected along the stream channel and surface tributaries on August 15, 2017.…”

“…As described by Briggs, Harvey, et al (2018), deuterium excess was used to identify discharging groundwaters that had experienced evaporation due to exchange with at least one upgradient groundwater flow‐through kettle pond. Several such ponds are hydraulically connected to the groundwater flow system that discharges directly to the Quashnet River (Barlow & Hess, 1993; LeBlanc et al, 1986) and are known to have substantial PFAS concentrations (Massachusetts Department of Health, 2019).…”

“…In this study, we integrate imaging of streambank discharges with thermal infrared cameras (e.g., Handcock et al, 2012), push‐probe streambed temperature measurements (e.g., McCobb, Briggs, LeBlanc, Day‐Lewis, & Johnson, 2018), and previously collected fibre‐optic distributed temperature data (e.g., Rosenberry, Briggs, Delin, & Hare, 2016) to guide sampling of discharging groundwater for geochemical, gas, isotopic, and PFAS analysis along 6 km of a coastal stream. The stream is home to an important managed native brook trout population, and we build directly on recent research from the same stream system showing that trout tend to favour local, oxygen‐rich groundwater discharges at the base of near‐channel hillslopes for spawning (Briggs, Harvey, et al, 2018). Brook trout release their eggs in the fall, which subsequently develop slowly over the winter in shallow streambed gravels (Cunjak & Power, 1986), increasing the potential for exposure to emerging contaminants in groundwater discharge compared to fish that spawn and hatch within the spring season.…”