Evaluation of the influences of dam release types, land use, and habitat affecting abundance, richness, diversity, and community structure of larval and juvenile fish

We systematically evaluated the influence of dams on downstream aquatic habitat and early life stage (ELS) fish at two spatial scales from epilimnetic and hypolimnetic discharges and made comparisons with a control stream. ELS fish exhibited a general increase in abundance, richness, and Shannon diversity with increasing distance from dams in both epilimnetic and hypolimnetic release types. Nonmetric multidimensional scaling (NMDS) indicated larval and juvenile fish communities were structured differently between epilimnetic and hypolimnetic releases, and habitat variables structuring those communities were more variable in epilimnetic releases than in hypolimnetic releases. Rapid changes occurred within the first 1100 m reach and a second more gradual gradient extended beyond the 5100 m sample reach. Generally, our findings agreed with that of the Serial Discontinuity Concept (SDC), but we believe future studies should be explicit in the systematic evaluation of the SDC, and further studies are required to understand the two recovery gradients that exist below impoundments.

Longitudinal trends in regulated rivers: a review and synthesis within the context of the serial discontinuity concept

Dams alter the geomorphology, water quality, temperature regime, and flow regime of lotic systems influencing the resources and habitat of fish, benthic invertebrates, and lower trophic levels. Since the inception of the river continuum concept and the serial discontinuity concept (SDC), biotic and abiotic impacts below impoundments have been the focus of many lotic studies. However, recovery gradients below dams are rarely examined in sufficient detail and no current synthesis of longitudinal impacts in regulated rivers exists. This understanding is needed to build ecological relationships in regulated rivers to inform environmental flows science and management. In this review, we provide evidence for SDC predictions on physical, chemical, and biological recovery in regulated rivers. Additionally, we determine how these changes are reflected in the benthic community. Our review suggests that two recovery gradients exist in regulated rivers: (1) a longer, thermal gradient taking up to hundreds of kilometres downstream; and (2) a shorter, resource subsidy gradient recovering within 1–4 km downstream of an impoundment. Total benthic invertebrate abundance varies considerably, with both increases and reductions observed at near-dam sites and varying in recovery downstream. Much of this variability stems from the degree of flow alteration and resource subsidies from the upstream reservoir. In contrast, benthic diversity is often reduced below dams irrespective of dam location and operation with little recovery observed downstream. The community at near-dam sites is largely composed of filter-feeding invertebrates which are quickly replaced downstream, while stoneflies are reduced below impoundments with limited downstream recovery. Despite a lack of formal testing, studies support SDC predictions. The SDC still provides a useful theoretical framework for hypothesis testing, and future studies should further expand the SDC to include empirical estimation within the context of the landscape.