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Secchi depths in lakes of Cape Cod National Seashore from 1996–2016 and relationships with morphometry, water chemistry, and housing densities

ABSTRACT Smith SM, Fox SE, Lee KD, Medeiros K, Plaisted HC. 2017. Secchi depths in lakes of Cape Cod National Seashore from 1996 to 2016 and relationships with morphometry, water chemistry, and housing densities. Lake Reserv Manage. 34:88–103. The landscape of Cape Cod (Massachusetts, USA) is scattered with numerous freshwater lakes (known as kettle ponds) that offer abundant recreational opportunities and provide important habitat for a wide variety of flora and fauna. The rapid urbanization of Cape Cod has been cause for concern regarding potential impacts on these waterbodies. Septic system delivery of nutrients to the ponds and atmospheric nitrogen deposition from fossil fuel combustion has been high for many decades in this region. These systems have also undergone significant shifts in pH and temperature due to regional changes in air quality and climate warming. All factors have the potential to alter pond trophic state and, consequently, water clarity. In this study, we examined August trends in kettle pond Secchi depths within Cape Cod National Seashore from 1996 to 2016. We further analyzed relationships by regression analysis between recent (2013–2014) measurements of Secchi depths and water quality constituents, physical features, shoreline housing densities, and the occurrence of gulls, fish stocking, and herring spawning. This provided a “snapshot” portrait of the current state of these ponds and their functioning. The results suggest that water clarity has remained unchanged or has even increased over the last 2 decades. Water clarity was negatively related to deep-water nutrient concentrations (total nitrogen and phosphorus), which themselves were negatively correlated with pond depth. The occurrence of gulls and anadromous and stocked fish populations had no apparent effect on water clarity, nor did any of the variables that described housing densities around ponds.