Seasonal dissolved inorganic nitrogen and phosphorus budgets for two sub-tropical estuaries in south Florida, USA

Abstract

Abstract. Interactions among geomorphology, circulation, and biogeochemical cycling determine estuary responses to external nutrient loading. In order to better manage watershed nutrient inputs, the goal of this study was to develop seasonal dissolved inorganic nitrogen (DIN) and phosphorus (DIP) budgets for the two estuaries in south Florida, the Caloosahatchee River estuary (CRE) and the St. Lucie Estuary (SLE), from 2002 to 2008. The Land-Ocean Interactions in the Coastal Zone (LOICZ) approach was used to generate water, salt, and DIN and DIP budgets. Results suggested that internal DIN production increases with increased DIN loading to the CRE in the wet season. There were hydrodynamic effects as water column concentrations and ecosystem nutrient processing stabilized in both estuaries as flushing time increased to > 10 d. The CRE demonstrated heterotrophy (net ecosystem metabolism or NEM < 0.0) across all wet and dry season budgets. While the SLE was sensitive to DIN loading, system autotrophy (NEM > 0.0) increased significantly with external DIP loading. This included DIP consumption and a bloom of a cyanobacterium (Microcystis aeruginosa) following hurricane-induced discharge to the SLE in 2005. Additionally, while denitrification provided a microbiallymediated N loss pathway for the CRE, this potential was not evident for the SLE where N 2 fixation was favored. Disparities between total and inorganic loading ratios suggested that the role of dissolved organic nitrogen (DON) should be assessed for both estuaries. Nutrient budgets indicated that net internal production or consumption of DIN and DIP fluctuated with inter-and intra-annual variations in freshwater inflow, hydrodynamic flushing, and primary production. The results of this study should be included in watershed management plans in order to maintain favorable conditions of external loading relative to internal material cycling in both dry and wet seasons.

Modeling ecosystem processes with variable freshwater inflow to the Caloosahatchee River Estuary, southwest Florida. I. Model development

“…The potential effects of altered discharge are heightened in sub-tropical estuaries with heavily modified watersheds and managed freshwater supplies (Childers et al, 2006;Petes et al, 2012;Buzzelli et al, 2013d). The Caloosahatchee River Estuary (CRE) in southwest Florida has a watershed characterized by extensive agriculture and urbanization, is influenced by regulated freshwater outflow from Lake Okeechobee, and contains valuable biological resources (Chamberlain and Doering, 1998;Doering et al, 2002Doering et al, , 2006SFWMD, 2010).…”

“…While monthly watershed concentration data were adequate for use as external drivers, data on submarine light, water column rate processes, and biogeochemical cycling at the sediment-water interface within the CRE were lacking. Despite its regional importance and suggested sensitivity to nutrient loading, it is unfortunate that phytoplankton and nutrient turnover in the CRE has not been quantified more extensively (Buzzelli et al, 2013d).…”

“…The CRE responds to external DIN loading by increasing rates of primary production, remineralization, and DIN concentrations (Buzzelli et al, 2013d). However, DIN accounts for only~30% of total N loading to the estuary with the dynamics of dissolved ON remaining largely unknown (Smith and Hollibaugh, 2006; Eyre Table 4 Results from sensitivity tests of Caloosahatchee River Estuary (CRE) simulation model.…”

“…Freshwater inflow to the CRE reflected the dry-wet seasonality of south Florida (Buzzelli et al, 2013c(Buzzelli et al, , 2013d. Releases of freshwater through S-79 in both dry and wet seasons followed the operational schedule to maintain pool elevations in the C-43 canal and Lake Okeechobee (SFWMD, 2010;Buzzelli et al, 2014a).…”

“…Variability in discharge on different scales directly affects flushing time, the input of colored dissolved organic matter (CDOM), and estuarine biogeochemistry (McPherson and Miller, 1987;Miller and McPherson, 1991;Bowers and Brett, 2008). Flushing time is one of the key modulators of allochthonous and autochthonous materials (Dettmann, 2001;Sheldon and Alber, 2006;Swaney et al, 2011;Buzzelli et al, 2013d). In fact, phytoplankton production and trophic transfer are directly influenced by freshwater inflow and flushing time (Lucas et al, 2009a;Phlips et al, 2012).…”