Condition and trends of estuarine water quality and seagrass in Cape Cod National Seashore: Northeast Coastal and Barrier Network, 2003-2022

Nagel, Jessica; Plaisted, Holly; Peck, Christopher; Duffy, Paul

doi:10.36967/2310108

This report summarizes conditions and trends in routine long-term water quality monitoring and seagrass data collected at Cape Cod National Seashore in Massachusetts from 2003 through 2022 as part of the NPS NCBN Vital Signs Estuarine Nutrient Enrichment monitoring program. This comprehensive program incorporates spatial surveys, repeated monitoring at trend stations, and continuous monitoring at a fixed logging station in an effort to establish overall water quality condition of the estuary as well as to identify potential trends or changes in condition over time. The monitored area included three units or strata, and all monitoring was conducted during a four-week summer index period. Monitored variables included dissolved oxygen concentration, chlorophyll-a concentration, attenuation of downwelling photosynthetically available radiation (PAR), salinity, temperature, turbidity, and pH. In addition, long-term monitoring of seagrass condition was conducted at permanently established transects in two reference beds. Water quality condition and trends were generally consistent between the spatial station surveys, the trend stations, and continuous logging station. Although chlorophyll-a concentrations and water clarity, as measured by PAR attenuation, were generally classified as being in fair to good condition throughout the majority of the park estuaries with improving or stable trends, poor conditions were still observed occasionally within the park over the timeframe between 2007 and 2021. Bottom dissolved oxygen concentrations exhibited significant improving trends and were consistently classified as being in good condition throughout the majority of the park estuaries. A significant increase in water temperature was observed in one stratum and across all trend stations over time, and summertime temperatures at the logging station, most trend stations, and in a large percentage of the estuary exceeded 25°C in several years. Significant declines in eelgrass in the park's reference seagrass beds between 2003 and 2022 suggest that conditions in the shallow and deep extremes are becoming less conducive for supporting eelgrass growth. Given that water temperatures in the park are increasing and that warmer temperatures may negatively affect both water quality and seagrass, the continued monitoring of estuarine nutrient enrichment vital signs and seagrass condition will be essential for managers to make informed decisions, develop effective mitigation measures, reduce management costs, and provide a means of measuring progress towards goals.

Condition and trends of estuarine water quality and seagrass in Cape Cod National Seashore: Northeast Coastal and Barrier Network, 2003-2022

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