American Water Resources Association 2016 ANNUAL WATER RESOURCES CONFERENCE November 14‐17, 2016 Orlando, FL Monday, Nov. 14 1:30 PM – 3:00 PM SESSION 7: Environmental Impacts of Sea Level Rise Phosphorus Storage in Coastal Sediments: Will Sea‐Level Rise Mobilize P and Elevate Coastal Fluxes? ‐ Andrea Pain, University of Florida, Gainesville, FL (co‐authors: C. Young, J. B. Martin) Phosphorus (P) is a limiting nutrient in many aquatic systems, especially in freshwater where iron oxide minerals sorb and sequester dissolved P. Saltwater systems lack this sedimentary P sink as sulfate reduction results in precipitation of Fe‐sulfide minerals limiting Fe‐oxide formation. Iron‐sulfur‐ oxygen redox reactions are enhanced at the freshwater‐saltwater interface of coastal aquifers, a region known as the subterranean estuary (STE), which discharges fresh water and solutes to coastal zones via submarine groundwater discharge (SGD). While SGD typically carries low concentrations of P due to sorption interactions with solid phases, particularly Fe‐oxides, we hypothesize that sea level rise will increase P fluxes from coastal sediments as sedimentary P storage decreases when saltwater intrudes sediments deposited in fresh water, precipitates iron sulfide, and removes the sedimentary Fe‐oxide P sink. We test this hypothesis across a salinity gradient in Fe‐oxide‐containing STE sediments from Indian River Lagoon, FL. Sediments have three distinct stratigraphic units: upper saline sediments dominated by iron‐sulfide minerals and elevated organic carbon (OC) content, a transitional layer of decreasing iron sulfide mineral content and low organic carbon content, and a deep layer characterized by quartz sand coated with Fe‐oxide and low OC content. Total P content of the upper layer is low (3.71.5 �mol/g) despite high total organic C content. The molar ratio of OC: total P (405342) is greater than the Redfield Ratio (C:P = 106:1), indicating that P remineralized from organic matter is preferentially lost from the sediment. P content of the transitional layer (1.71.8 �mol/g) is similar to the upper layer despite low OC content. OC: total P ratios lower than the Redfield Ratio value (1917) suggest that P is stored in mineral P reservoirs rather than organic P. The deep layer is characterized by elevated P content, with the highest concentrations in the shallowest interval of this unit (55 �mol/g) and generally decreasing with depth. Low overall OC content and low OC:P ratios (53), combined with high Fe‐oxide content, suggest that Fe‐bound P constitutes a large portion of total sediment P, and is highest in the shallowest portions of this interval. If P contained in this layer is relic P from when sediments were deposited in freshwater (fluvial?) systems at lower sea level, remobilization would represent an additional P source above that recycled from lagoon sediments, thus the lagoon P budget could be significantly altered. Relationships between P content, Fe‐oxide abundance, and organic matter content suggest that saltwater intrusion could mobilize P, thereby increasing P fluxes to coastal zones, as iron speciation in sediments shifts from Fe‐ oxide to Fe‐sulfide minerals. This increase in P fluxes could be exacerbated by accelerating sea level rise, thereby altering P‐limited coastal ecosystems. Florida's Spring‐Fed, Coastal Rivers: Past, Present and Future ‐ Thomas Frazer, University of Florida, Gainesville, FL (co‐authors: C. A. Jacoby, S. K. Notestein) Research over the last decade indicates a precipitous decline in macrophyte abundance in spring‐fed, coastal rivers along the west coast of peninsular Florida. Of particular concern is the decline of native species, such as American eelgrass (Vallisneria americana) and strapleaf sagittaria (Sagittaria kurziana). In fact, S. kurziana appears to have been largely extirpated from many of these systems. Macrophyte loss often has been attributed to increased nutrient loading and, in fact, concomitant increases in macroalgal abundance and periphyton loads in several spring‐fed, coastal rivers are consistent with a simple eutrophication progression scheme. However, altered salinity regimes due to declines in freshwater discharge, shoreline modifications, and increases in the frequency and intensity of tropical storm events likely have played an important role in the deterioration of these systems. In the Kings Bay/Crystal River system, for example, losses of native vegetation and concurrent increases in non‐native and nuisance flora can be linked to both chronic and acute changes in salinity. The longer‐term, negative consequences of sea‐level rise on the biology and ecology of Florida's spring‐ fed, coastal rivers are legitimate reason for concern. Freshwater flow in each of these systems originates as groundwater discharge emanating from spring vents that are fixed in geographical space. As sea level rises, the freshwater portions of these systems will become compressed, and conditions will become less favorable for freshwater macrophytes and the faunal assemblages that are supported by these important structural habitats. For example, recent research carried out in the Chassahowitzka and Homosassa rivers indicates that losses of native submersed aquatic vegetation (freshwater macrophytes in particular) coincide with reductions in the overall biomass of freshwater fishes and loss of key species. Such changes are likely to have profound consequences for the ecological health and integrity of these coastal, spring‐fed systems. A 65‐Yr Migration of Sea‐Level Rise Hot Spots Along the U.S. Atlantic Coast ‐ Arnoldo Valle‐
Levinson, University of Florida, Gainesville, FL (co‐authors: A. Dutton, J. Martin) Sea‐level rise (SLR) has accelerated in a "hot spot" along the north of Cape Hatteras over the past several decades, including an abrupt rise of ~13 cm in 2009‐2010. This regional acceleration in SLR has been attributed to weakening in Atlantic Meridional Overturning Circulation (AMOC), although this causal link remains debated. We document a striking shift in the pattern of SLR along the U.S. Atlantic coast during 2011‐2015, whereby SLR decelerated north of Cape Hatteras and accelerated south of the Cape to >20 mm/yr, despite continued decline in AMOC strength. We show from 95‐yr tide gauge records that similar short‐lived, rapid SLR intervals have occurred repeatedly over ~1500‐km stretches of coastline. These records reveal that hot spots of SLR have migrated northward since the 1940s and suggest that the reappearance of the hot spot in the southeastern U.S. may represent a re‐initiation of a ~65‐year cycle. This observation challenges the notion that accelerated SLR north of Cape Hatteras will be a persistent feature associated with a weakening AMOC and instead suggests that hot spots of SLR impact the entire U.S. Atlantic coast in a defined spatio‐temporal pattern. Causes for this pattern are related to the superposition of multiple oceanic and atmospheric circulations processes that occur with different frequencies. The regional expression of SLR hot spots documented here is a key factor in determining coastal vulnerability in the context of continued global mean sea‐level rise and should be captured in global climate models of regional sea‐level change. Forecasting Coastal Forest Die‐Off in the Lower Suwannee Refuge: Influence of Climate Drivers and Island Characteristics ‐ Katie Glodzik, University of Florida, Gainesville, FL (co‐
author: D. Kaplan) Sea level rise and reduced freshwater discharge are driving higher coastal salinity along the Big Bend of Florida, causing widespread coastal forest die‐off and replacement by salt marsh. Coastal forest is dominated by cabbage palm (Sabal palmetto) and red cedar (Juniperus virginiana), which can tolerate periods of moderate salinity, but become stressed from sustained higher salinity. Previous studies that monitored research plots revealed that islands experiencing die‐off tended to have more frequent tidal flooding and be at lower elevations, though instances were found of low‐elevation islands supporting healthy vegetation. Higher die‐off rates also occurred during drought years, suggesting the importance of freshwater influence. While it is generally known that higher salinities are causing coastal forest die‐off, there is limited information on how sea level, local rainfall, river discharge, and groundwater seepage interact to drive groundwater salinity, and whether these drivers vary in different areas. To improve planning and management of coastal refuges, better quantification is needed of how freshwater influence affects the elevation at which sea level rise causes coastal forest die‐off. This study examines how island characteristics (elevation, distance to coast) and hydrological variables (tidal level, river discharge, local rainfall, and indirectly, groundwater influence) affect coastal forest die‐off in the Lower Suwannee National Wildlife Refuge. A remote sensing analysis was completed to compare island vegetation health with elevation and distance to coastline. Four wells for groundwater level and salinity monitoring were installed in islands that have expected vegetation health status based on elevation and distance to coast, and four were installed in forest islands that diverged from this relationship (e.g. relatively low‐ elevation islands with healthy forest). Groundwater salinity time series were compared to time series of tides, river discharge, and local rainfall to answer 1) do coastal forest islands get freshwater by retaining rainfall or does it appear to be from groundwater discharge and 2) what is driving elevated salinities in stressed areas?