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.
Spatial and Temporal Variability of Macroinvertebrates in Spawning and Non-Spawning Habitats during a Salmon Run in Southeast Alaska