Temporal Variability of Water Quality Parameters at the Elkhorn Slough Estuary using Wavelets

In recent decades, significant temporal variability has been observed in water quality parameters, especially in coastal settings. To ensure sustainable water resources management, it is important to document the dominant scales of variation of these parameters and identify the underlying factors causing these variations. These variations are typically governed by a complex interplay of factors including, but not limited to, anthropogenic controls (e.g., a culvert), climatic conditions and hydro-biogeochemical processes. To decipher these underlying controls, there is a significant need for advanced statistical approaches. Here, we use wavelet analysis on an extensive water quality parameter dataset available from the Elkhorn Slough estuary, located in Monterey Bay, California, for over 15 years. Results from the wavelet analysis indicate that ammonia, nitrite, nitrate, and salinity have a dominant 12-month frequency at the head of the estuary. An exception to the rule was orthophosphate at this location, which showed multi-scale frequencies, varying from 4-32 months in addition to the commonly observed annual-scale variation. While the parameters showed consistent 12-month frequency at the head of the estuary, salinity, ammonia, and nitrite exhibited multi-year frequencies at the mouth. In conclusion, mixing of different source waters in a tidally-controlled estuarine environment can result in significant spatial and temporal variability in water quality parameters.