AP Ecological Flows Phase II Report

Developing Coastal Plain Ecological Flow Guidance in the Albemarle-Pamlico Basin: Phase II Trent River Pilot Study

Ecological flows of inland freshwater rivers are those that maintain ecological integrity and are embodied in policies for planning and management of water withdrawals. This report focuses on how low flows may alter water quality, habitat distribution and community structure. Specifically, we describe a study on the Trent River, NC, in the context of broader efforts on policy. Rivers within coastal plains present issues regarding ecological flows different from those within piedmont and mountain watersheds. These issues are addressed with special reference to the Albemarle Pamlico Sound region.

Low flow conditions, their causes, and consequences are linked to elevation, land and water use, and meteorology. Elevation above sea level is critical to how ecological flows are understood. In the mountains, piedmont and upper coastal plain, low flow is generally inferred to alter habitat through vertical changes in river stage. A decrease in water level changes the distribution of riverine habitats and the communities of fishes, invertebrates, and other organisms. However, in the lower coastal plain, sea level begins to affect stage and habitats as the river approaches base level. Low flows in this region are less likely to have a major influence on water levels but may influence water quality due to the increased likelihood of saltwater intrusion during low flow conditions along tidal reaches. Salinity, dissolved oxygen, and temperature are water quality parameters that are likely to alter habitat. Further, human relationships to waterways and riparian land use may change as elevation approaches sea-level.

We utilized a transdisciplinary approach to evaluate the influence of low flows on coastal plain river ecosystems. The disciplines include hydrology, geography, ecology and sociology. This pilot study was conducted on the Trent River, a tributary to the Neuse River Estuary. The Trent River was selected by the APNEP Ecological Flows Action Team as an ideal location for a pilot project due to the availability of flow, water quality and ecological data; a diversity of land use and water users; its proximity to the coast; and interest from the NC Department of Environmental Quality. Historical data on flows, water levels, water quality and fauna have been assessed relative to drought periods causing low flows. These data supplement an intensive longitudinal study of water levels and conductivity during the drought of 2021-2022.

The geospatial component of the study provided georeferenced context for other components. The hydrology component monitored flows, water levels and conductivity along the Trent River for nearly two years. Head of tide was identified and was upstream of measured saltwater intrusion. Low flows along the Trent River were observed to (1) disconnect water level from discharge and (2) promote inland saltwater intrusion. Downstream effects from estuaries and neighboring rivers on coastal plain river dynamics are underappreciated and shown to be important. The ecological component, thus far, has identified potential salinity thresholds of impact from the literature. Specifically, we have estimated consensus thresholds for wetland trees and fish species when experiencing repeated exposure to oligohaline or saltier conditions. For wetland trees a threshold of 2 ppt begins to impact the most sensitive species, life history stages, and ecological processes. A threshold of 6 ppt begins to show mortality and significant other detrimental impacts to plant 2

physiological water-use processes. Fish habitat was estimated to be optimum for most appropriate species when salinity was < 2 ppt and suitable when < 5 ppt.

The human dimension component assessed ecological policies within North Carolina and the United States. Further, few studies have incorporated stakeholder preferences into frameworks designed to determine policies for management of low flows. Surveys were distributed among eastern North Carolina water users to determine perceptions of change in flow conditions over time, consumptive and non-consumptive water uses, and preferences regarding potential policy actions. This transdisciplinary approach is considered as a model for addressing not only ecological flows within coastal plain rivers and watersheds but other socio-ecological issues. In light of growing water demands, land use, and climate change, there is an increased risk that low flow conditions can impact coastal aquatic ecosystems. Broader efforts are needed to organize stakeholders to develop support and strategies to advance system-wide policies related to ecological flows for the Albemarle-Pamlico Drainage Basin. The Albemarle- National Estuary Partnership (APNEP) is positioned to be a leader in expanding efforts not only on ecological flows in coastal plain rivers but also broader issues of water management across the spectrum of flows.