Seasonal dynamics of dissolved, particulate and microbial components of a tidal saltmarsh-dominated estuary under contrasting levels of freshwater discharge

Abstract

Tidal Spartina-dominated saltmarshes and estuaries on the Southeast US coast are global hotspots of productivity. In coastal Georgia, tidal amplitudes and saltmarsh productivity are the highest along the Southeast US coast. Coastal Georgia is characterized by a humid subtropical seasonal climate, and inter-annual variability in precipitation, and freshwater discharge. The 2012–2013 timeframe encompassed contrasting levels of discharge for the Savannah River, a major Georgia river, with a 4.3-fold greater discharge in summer 2013 relative to summer 2012. In situ measurements of temperature, salinity, precipitation and Secchi depth, and water samples were collected weekly at high tide throughout 2012 and 2013 from the Skidaway River Estuary, a tidal saltmarsh-dominated estuary in coastal Georgia influenced by Savannah River hydrology. The effects of elevated discharge on the seasonal trends of water column components were evaluated. The shift from low discharge (2012) to high discharge (2013) led to decreased salinity in summer 2013, but no significant increases in inorganic nutrient (NH4, NOx, SiO2 and PO4) concentrations. Dissolved inorganic carbon (DIC) concentrations decreased, and DIC stable isotopic signatures (δ13C-DIC values) were depleted in summer 2013 relative to summer 2012. In 2013 dissolved organic carbon (DOC) concentrations, chromophoric and fluorescent dissolved organic matter (DOM: CDOM, FDOM) intensities, specific UV-absorbance (SUVA254) and relative humic-like fluorescence were all higher than in 2012, indicating that, as discharge increased in 2013, estuarine water became enriched in terrigenous DOM. Secchi depth and particulate organic carbon (POC) and nitrogen (PON) concentrations displayed clear seasonal patterns that were not significantly altered by discharge. However, δ13C-POC and δ15N-PON isotopic signatures indicated higher terrigenous contributions at elevated discharge. Discharge influenced cyanobacterial composition, but did not affect total abundance of phytoplankton (<52 μm) or chlorophyll-a concentrations, a proxy for phytoplankton biomass. Phytoplankton community dynamics were primarily seasonally-driven. Bacterioplankton abundance and community composition, based upon flow cytometry, were affected by discharge, possibly due to decreased salinity and/or increased inputs of terrigenous DOM. Seasonal patterns in inorganic nutrient, POC, PON and chlorophyll-a concentrations, and Secchi depth were not significantly influenced by the 2013 increase in discharge. For other components, most notably δ13C-DIC values, DOM and bacterioplankton, the influence of increased discharge in 2013 was superimposed upon their seasonal patterns. This study showed that in addition to tidal mixing and in situ saltmarsh and estuarine production and removal processes, the level of riverine freshwater discharge impacted the quantity and character of many water column components in this tidal saltmarsh ecosystem.