Spatiotemporal variability of dissolved oxygen in response to morphological state in a central California coast bar-built estuary

Abstract

Bar-built estuaries (BBEs) or intermittently closed/open lagoons (ICOLs) are common to coastal regions experiencing episodic river flows and seasonal precipitation changes along the US West Coast. Unique to Mediterranean climates, BBEs have a hydrologic mixing regime and variation in water chemistry characteristics resulting from seasonal breaches and closures of their mouth and watershed inputs. This study observes spatiotemporal variability in dissolved oxygen (DO) in a small bar-built estuary in Carmel, CA during variable morphological states. For this study, field measurements were collected using dissolved oxygen loggers during a seasonal transition from fully open to fully closed states to determine the spatiotemporal variability of dissolved oxygen within the three main branches of Carmel River Lagoon. The data collected spanned a 6-month (February to August 2020) period, covering several transitions from fully open to intermittently closed to fully closed mouth state. This study did not document any periods within the lagoon when dissolved oxygen concentrations were uniformly below water quality objective levels (5 mg/L) suggesting that during the entire period of this investigation, portions of the lagoon supported high oxygen conditions while also supporting a variety of other water chemistry characteristic zones, unique to BBEs. The dissolved oxygen fluctuations were closely tied to the temporal effects of environmental forcing (river discharge) on the seasonal transition of river mouth morphological state from fully open to fully closed. The main conclusion is that during open mouth states, relative DO variability in the bottom of the main channel is low and more similar to surface waters, indicating mixing, and during closed mouth states vertical location in the water column matters more. Furthermore, variability between sites suggests single point observations of DO cannot accurately represent the distribution of DO within a BBE.