Coastal SEES: Sea-level change and thresholds for coastal water sustainability

Coastal SEES: Coastal water sustainability: Sea-level change and thresholds

I. INTRODUCTION

This proposal addresses thecritical question: To what extent are water resources (both fresh and salt) in coastal zones sustainable in the face of accelerating sea-level rise and population growth? From this question, we develop four specific hypotheses (Section III) to be tested using a work plan (Section IV) of collaborative interdisciplinary field observations, experiments, laboratory analyses, and modelingand broader impacts (Section V)includingdevelopment of an interdisciplinary STEM workforce and contributions to site-specific coastal management.Our question, hypotheses, and work plan arise from results of a Track 1 Coastal SEES planning project (Section II) thatallowed us toassemble thismultidisciplinary teamof PIs with excellent working relationships and communicationabilities.

Our question stems fromuncertaintiesof how sea-level change impacts coastal water resources at various time and spatial scales. Uncertainties arise from multiple linked physicochemical, biological and human processes occurring at coastal zones (Fig. 1). These linked processes require interdisciplinary research to respond and adapt tosea-level rise at optimal times, since response and adaptation may become prohibitively expensive or impossible to accomplishonce changing conditions pass certain thresholds (abrupt changes in rates or conditions).Our project aims to address severalimportant questions through continued interdisciplinary collaborations initiated with our Coastal SEES Track 1 planning grant. These questions include:

  • What arepotential rates of sea-level changeand how will theyimpact water resources for human andecological services?
  • What processes have thresholdsthat when passed will degrade water resources?
  • How willcrossing thresholds impact natural and built coastal environments?
  • How do thresholds vary with hydrologic characteristics of coastal aquifers?

II. Results from prior NSF support

A. Martin, Ogram, Peng, Valle-Levinson, OCE-1325227, $441,125, 8/15/2013-7/31/2015 (plus 1-yr no-cost extension),“Coastal SEES (Track 1): Planning for hydrologic and ecological impacts of sea-level rise on sustainability of coastal water resources”.

Intellectual Merit: This project (hereafter referred to as CS1 for “Coastal SEES Track 1”) accomplished interdisciplinary field observations, laboratory analyses, and modeling that were used to assess how sea-level change affectshuman communities, water resources, coastal hydrodynamics, microbial community structures and functions, and biogeochemical reactions at various time scales (Fig. 2). Theprojectfocused on two coastal settings representing end-member subterranean estuaries (STE)1: the east coast of Florida, USA and the east coast of Quintana Roo,Mexico (Yucatan peninsula). Both sites discharge fresh water to coastal lagoons: the Florida STE from a granular siliciclastic aquifer and the QuitanaRoo STE from a carbonate karst aquifer (Fig. 3).These two regions, which are sites of recent rapid population growth,also constitute the field sites for this proposed project (hereafter referred to as CS2 to differentiate it from results derived from CS1).

Our CS1 results show that hydrogeology and biogeochemical reactions at both sites depend on aquifer characteristicsand short-term (tidal to decadal) sea-level variations. In Quintana Roo (karst STE), fresh and brackish water dischargevia conduits (water-filled caves), but reverse flow (recharge) occurswhen sea level rises above a threshold of 0.08 m above the average value of our fortnightly observation periods (Fig. 4)2. During recharge, oxygen-rich lagoon water flows to the aquifer and catalyzes biogeochemical reactions, while during discharge,reaction products, including nutrient-rich water, flows to the lagoon. Switching from discharge to recharge was modulated by wind set up and spring-neap tidal cycles2-4.Biogeochemical reactions reflect changes in microbial community structures(Morrison et al., in review; Bae et al., in press) although understandingcommunity functionsrequires additional samples5.We predict recharge-discharge timing and magnitude will change as water demand,which has risen from 29 to 873 x 106 m3 between 1980 and 2012, increases with population growth (ref??- Deng and Peng, Chao and Peng in prep??).

In the granular aquifer (Florida), the location of the seepage face (the coastal zone discharging fresh water6) has shifted shoreward by tens of meters as the lagoon flooded following the Last Glacial Maximum7. Data from CS1 extendprevious observations to nearly 10 years and show ~5 m (~25%) shoreward migrationof the seepage face (Fig. 5). We speculate this shoreward shift reflectsacceleration of sea-level rise by about an order of magnitude to rates of ~20 mm/yr over the past 5 years throughout the South Atlantic Bightbased on tide gauge data8.

Biogeochemical reactions are similar at the two study sites and include enhanced organic matter remineralization, carbon cycling9, altered N speciation10, sources of P from organic matter and carbonate minerals2, reduced metal oxides, sulfate reduction and sulfide oxidation7,11,12, with resulting pH changes, mineral dissolution and changing sources and sinks of P2. These similarities occur regardless of time and spatial scale differences in STE responses to sea-level change (e.g., Fig. 3), suggesting general results about sea-level impacts on coastal zones can be obtained from these sites.Publications[JBM1]: To date, 4 abstracts and meeting presentations5,9,10,13others?), and 4 papers have been published, submitted or in preparation for the peer-reviewed literature2-4,8Dengand Peng, Chao and Peng in prep. others?).[JBM2]

Broader Impacts:CS1 integrated interdisciplinary activity at the University of Florida (UF) centered on coastal water resources and sea-level rise. Coupling CS1 with aUF Water Institute Graduate Fellows (WIGF) Programsupportedthe research of 10 PhD-level graduate students, one post-doctoral researcher, and two additional faculty members. Participants represent five departments (Geological Sciences, Coastal Engineering, Soil and Water Science, Urban and Regional Planning, Wildlife Ecology) and four colleges (Liberal Arts and Sciences; Engineering; Agriculture and Life Sciences; Design, Construction and Planning), reflecting the range of disciplines involved. One of the additional faculty members (Dutton) is aco-PIand helped develop this proposal withher prior NSF support (Section IIB).

Our work in Quintana Roo established synergistic activities with Mexican colleagues at CINVESTAV-IPN and Centro de InvestigacionCientifica de Yucatan, in Merida, and at Universidad Nacional Autonoma de Mexico and Parque Nacional Arrecife in Puerto Morelos, including collaborative work with two faculty there and 4 of their graduate students (2 PhD and 2 MS)2,13. Our work in Indian River Lagoonprovides estimates of benthic nutrient fluxes from the STE in support of the St. Johns River Water Management District(one of five Florida water management districts) efforts to prepare a nutrient reduction model to manage harmful algal blooms.

B. Dutton Sole-PI; EAR[JM3]-1155495, $314,258; 4/1/12 – 5/31/16, “Towards a Global Reconciliation of Last Interglacial Sea-Level Observations”

Intellectual Merit: Given the current inability to project dynamic instability of ice sheets under future warming scenarios, the behavior and dynamics of polar ice sheets during past warm periods provide indispensable empirical constraints. This project aims to reconcile observations of the magnitude of peak sea level during the last interglacial period and sea-level minima associated with glacials MIS 2 and 6 through fieldwork to survey and collect samples, U-Th dating of corals, and glacial isostatic modeling. Publications: Three papers have appeared in the peer-reviewed literature (refs);another two student-led manuscripts are in preparation(refs); at least three more are anticipated; 14 abstracts (6 posters/8 talks, references) presented by the PI and a PhD graduate student at domestic and international conferences.

Broader Impacts:The grant has supported three graduate students, one undergraduate researcher, a class of students who participated in collection of field data in the Bahamas, a video exhibit and a new exhibit on sea-level change being developed in collaboration with the Florida Museum of Natural History. Publications in QSR and Science received significant media attention. The Science paper was picked up by more than 25 media outlets globally including Reuters, The Guardian, and The Washington Post. Research from this grant has been featured twice by NSF (on their home page in Jan-2015 and as the top news story for Science360 in July-2015). This media attention directly led to further engagement in public outreach to communicate the research results, including involvement in an upcoming feature film about climate change and a documentary series on climate change being produced by National Geographic (with an episode focused on sea-level rise in south Florida), contributing to an in-depth piece for the New York Times Magazine, and being consulted in-person by U.S. Senator Bill Nelson (D-FL) regarding sea-level rise in Florida.

III. HYPOTHESES AND SIGNIFICANCE

A. Hypotheses to be tested

Hypotheses proposed here center on theoverarching hypothesis that sustainability of coastal water resources depends on rates of sea-level change, consumption and degradation of water resources, biogeochemical transformation at the fresh-salt water interface, and physical connections between fresh and salt water.We breakthis general hypothesis into specific hypotheses to be tested by our interdisciplinary work plan (Section IV) of linked and simultaneous observations, experiments, models, and analyses. Our work plan describes specific objectives designed to test each of the following hypotheses, both individually and through synthesis of the each objective’s outcomes.Specific hypotheses and tests are:

Hypothesis 1:Sea-level rise and coastal population growth will degrade coastal water resources at dissimilar spatial and temporal scales. This hypothesis will be tested through comparing extant observations and projections of coastal development and population growth with evaluations of potential rates of sea-level change based sediment archives during the last interglacial period ~125,000 years ago (centennial to millennial rates), observations from tide gauge data (decadal rates), and direct observations(seasonal, storm and tidal scales).

Hypothesis 2:Water resources will degrade abruptly when thresholdsin population densities, sea level, water table elevationsare crossed. This hypothesis will be tested through high-resolution observations of changes in sea level and inland water table at intra-tidal to seasonal time scales; analyses of the impact ofchanging levels on salt and water exchanges between coastal and aquifer water in two end-member types of STEs; measurements of exchange effects on solute (e.g., C, N and P) concentrations; and compiling, modeling, and comparing data related to inland fresh water consumption relative to changes in water table elevations and population growth.

Hypothesis 3: Exceeding thresholdswill alter microbial community structures and functions, nutrient delivery, and salt fluxes; degrade potable water resources; increase cost of human adaptation; and reduce efficacy of adaption measures. This hypothesis will be tested by compiling water resource usage data and measuring water table and sea-level variation at seasonal and shorter time scales. These data will becompared with simultaneously measured changes in microbial communities and functions, chemical compositions of water, shifts in hydrodynamic behaviors, and possible contamination from the built environment. These comparisons will be made in the framework of past rates of sea-level change based on sediment archives and projections of future sea level and water usage.

Hypothesis 4: Crossing thresholds depends on hydrologic characteristics of the coastal aquifers comprising the STEs. This hypothesis will be tested by comparing data from two distinct STEs, both in rapidly developing settings. One STE is composed of granular siliciclastic sediments with diffuse flow, and the other STE is composed of variably lithified carbonate rocks and sediments with conduit flow (Fig. 3).

B. Scientific Significance

Eustatic sea level is rising at ~3.2 mm/yr andprojections suggest that if the rate accelerates as expected, sea level will be0.26 to 1.90 mhigher than now by 210014-16. Rising sea level represents a slow-moving problem, punctuated by short-term disasters (e.g., Hurricanes Katrina and Sandy) for the more than 1 billion people who live along coasts17. Particularly vulnerable areas include densely populated regions of Southeast Asia, Egypt, and sub-Saharan countries18 and the US where ~123 million people (39% of the population) live in coastal counties19. Although attention-grabbing events such as storms, storm surge, flooding, and increased erosion (NRC, 2007) are clear threats, less attention is focused on threats to coastal water resources (both fresh and salt) from rising sea level and increased population, which may limit coastal-community sustainability prior to inundation. Threats to fresh resources include over pumping, waste disposal, and related effects of increasing coastal populations20,21. Threats to salt water resources, which support coastal ecosystems and economies, include changed solute fluxesfrom reduced submarine groundwater discharge 22,23and altered compositions following reactions caused by salt water intrusion2.

Rates of sea-levelrise vary through time as shown by sea-level reconstructions that span timescales of 10-105 years e.g.24,25 and direct observations based on tide gauges and satellite altimetry14.Sea level rose 3 meterson ecological time scalesbased onspectacular successions of reef faciesat Xcaret in Quintana Roo from the last interglacial26,27. A similar rapid rise is shown by reef exposure surfaces in the Bahamas and Florida, but these sites suggest an ephemeral sea-level fall of at least 1 meter preceded the rise. These rapid changes in sea level reflect theinstability of polar ice sheets during warm climates28,29. Instabilities also appear as meltwater pulses, e.g.30 assea-level rises of 1 to several meters over 102 to 103 yearsduring retreat of Northern Hemisphere ice sheets over the past 20,000 years. The potential for futurerapid sea-level riserates is shown by the recent finding that the West Antarctic ice sheet is susceptible to collapsee.g.31. Understanding potential rates of sea-level change is thus critical to develop adaption strategies for coastal ecosystems and human communities.

Recent accelerationsof sea-level rise are shown by tide gauge and satellite altimetry data, which reveal a change from a global mean rate of ~1.2-1.9 mm/yr between 1901 and 1990to ~2.8-3.7 mm/yr from 1993-201032. Rates also vary spatially; tide gauge data show ~5 mm/yr rise along the Mid-Atlantic Bight over the past couple of decades33-36. These rates, which are in excess of the global average, reflectwind forcing, deceleration of the Gulf Stream, and changes and/or divergence in the Atlantic Meridional Overturning Circulation37-39. Rapid sea-level rise also began recently in theSouth Atlantic Bight,with tide gauges showing rates of ~18 mm/yrsince 20118,40. These rates may reflect changes to the Atlantic Multi-decadal Oscillationand slowing of the Florida current41,42.Temporal and spatial variations in sea-level riseshould alter biogeochemical processes in STEs and impact sustainability of coastal water resources, depending on hydrodynamic connections between coastal and aquifer waters.Rates of sea-level change and absolute elevationsthus represent important thresholds that,when exceeded, could permanently degrade coastal water quality.

Human behavior and development are going to be impacted by these variably rates of sea level change. Human behavior can also alter natural coastal processes. Consequently, understanding rates of sea-level change and natural coastal processes is required to develop predictive models, translate these models to policy, and develop adaptation strategies20. As coastal populations grow and develop coastal lands, water consumption is expected to increase as rising sea level shrinks water availability. Rising sea level and population growth also complicate the ability to treat storm and waste water. Balancing water demand and supply is thus critical and will require understanding how potential thresholds of sea level may affect coastal human communities as thresholds in water demand are reached. Results of natural process studies when linked to human-induced thresholds of water demand will also have to be interpreted based on past, current, and projected rates of sea level change, whether the rates are accelerating or decelerating, how rates control hydrodynamic and biogeochemical processes in the coastal systems.

Sea level change will first affect hydrodynamic and biogeochemical processes in STEs1, locations critical to sustainability of coastal water resources. They receive the first impacts from salt water intrusion as sea level rises21,43 andimpacts will bemodified by changing elevations of the groundwater table44,45Menning et al, 2015). Subterranean estuaries provide a source of ecologically important solutes including nutrients46-49 and metals50-52 to coastal zones.Nutrient fluxes may decreasewithsalt water intrusion or may increase as inland vadose zones shrink, limiting waste disposal options.Salt water intrusion should alter microbial community distributionsand functions in STEs, thereby altering biogeochemical reaction andnutrient fluxes49,53-55.Microbial community functions should be affected as dissolved oxygen (DO) and dissolved organic carbon (DOC) concentrations, compositions, origins, and lability change9,53,54,56-58.Organic carbon remineralization will alter compositions of the greenhouse gases carbon dioxide (CO2) and methane (CH4), nutrients including N and P, and toxins such as sulfide59. These changes willnegatively impact coastal economies, for example fromdiminished fisheries and tourism, as estuarine water quality degrades. Each of these processes may change abruptly, and possibly permanently, as sea level and water use surpass unknown thresholds.

The multiple natural and anthropogenic processes that impact coastal water resources (Fig. 1) require collaborations between natural and social sciences to develop adaptation strategies20.The challenge is to make observations, use proxy information, and develop models to understand rates of sea-levelchange, its control on coastal hydrodynamics and hydrogeology, its effect on biogeochemical reactions in STEsfrom exchange and mixing, how it may degrade water quality and quantity for human and ecological use, and how human behaviors impact and respond to these processes (Fig. 1). Resolving this challenge should provide pathways toward sustaining coastal water resources, the primary goal of this proposal.To reach this goal, our project integrates biogeochemistry, microbiology, hydrology, coastal hydrodynamics, and geology with social sciences including economics and regional planning to evaluate relationships between changing sea level, human activities,and coastal water resources, and to develop pathways toward sustainable coastal water resources.