Drought Impacts to Coastal Estuary Ecosystems in the U.S. Caribbean
Learn about the impacts of drought on coastal estuary ecosystems in the U.S. Caribbean below.
Authors: Brent Murry (USFWS), Miguel Garcia-Bermudez (USFWS), Shelley Crausbay (Conservation Science Partners), Kate Malpeli (National Climate Adaptation Science Center, USGS)
Click here to download the 2-page fact sheet with graphics, or read below for an extended version of the fact sheet.
Introduction
The topography of Puerto Rico and the U.S. Virgin Islands (USVI) is characterized by steep terrain and short distances to the sea. This means that freshwater runs off the islands quickly, coming into contact with seawater in coastal estuaries. The physical characteristics of estuaries change as the tides rise and fall, creating a wide range of habitats that support diverse plants and wildlife, including economically and culturally important native species such as cetí and land crabs, as well as game fishes such as snook and tarpon. These ecosystems are already heavily threatened by human activities such as urbanization, increased sedimentation, and pollution. Changing climate conditions, such as more frequent and severe drought, pose an additional stressor. Because rivers in Puerto Rico and temporary streams (known locally as “ghuts”) in the USVI feed the coastal estuaries of the U.S. Caribbean, changes to streamflow can impact estuaries and the wildlife they support. For example, during prolonged periods of low flow, withdrawals from the Espiritu Santo River, which feeds into the Espiritu Santo estuary in northeast Puerto Rico, can reach 100% of instream flow and the river can run dry (Benstead et al., 1999). This reduction in the amount of freshwater entering the estuary can increase salinity levels, altering habitat conditions and leading to declines in the richness and abundance of freshwater species (Smith et al., 2008).
Spatial Context
Located at the freshwater-saltwater interface, drought can impact the amount of freshwater that enters estuaries, thereby altering their composition. The amount of saltwater in estuaries is already increasing due to sea-level rise. This saltwater meets freshwater that has its origins in both surface water (e.g. rivers, streams, and overland flows) as well as groundwater. Decreases in rainfall result in decreased freshwater entering estuaries due to (1) reduced surface flows; (2) reduced aquifer recharge and groundwater flows; and (3) increased human withdrawals - which further drive reductions in surface and groundwater flows. Changes in the distribution and concentration of saltwater can have both short- and long-term impacts on mangroves and other keystone wetlands plant species, with subsequent impacts to numerous ecosystem services such as water filtration and storm surge protection.
Temporal Impacts
Short-term Impacts
The primary impact of drought on estuaries in the region will be reduced inflows of freshwater. In the short-term, increases in estuary salinity will increase the mortality of wetland plants, with subsequent impacts on the important services these ecosystems provide – such as nutrient up-take, sediment sequestration, water quality, fish nursery habitat, and bird nesting habitat.
- Wetland Function: As wetland species that are less salt-tolerant die, those that are more tolerant will take their place. This will change the ways in which wetlands function, but it’s possible that certain ecosystem services may be preserved (e.g. rates and amounts of sediment sequestration, nutrient cycling, and habitat heterogeneity).
- Habitat Changes: Mangroves are salt tolerant, however the degree of tolerance varies among species. For example, black mangroves are very sensitive to changes in hydrological regimes, and both drought and flooding can cause widespread mortality (Laboy et al., 2002).
Long-term Impacts
Changes in dominant wetland plants that provide the primary “structure” of estuaries will lead to subsequent changes to the invertebrate, fish, and bird communities, as well as ecosystem services.
- Long-term periods of drought will facilitate the transition of dominant wetland plants from freshwater-dependent species to brackish water, and eventually to marine-tolerant species, dramatically affecting habitat quality for fish and birds.
- Mangrove ecosystems could be converted to salt marsh or mud flats.
- In Puerto Rico’s Jobos Bay Estuary, decreased groundwater levels and high salinity levels have been recorded, affecting all species of mangroves (Laboy et al., 2002).
- Invertebrate communities in freshwater estuaries tend to be dominated by larval and adult insects, whereas marine estuaries tend to be dominated by crabs and shrimp. Similarly, freshwater and brackish estuaries tend to support culturally and economically important native species (e.g. cetí and land crabs), as well as game fishes such as snook and tarpon. These systems will change to dominance by marine species such as jacks, barracuda, and puffers.
Cross-Sector Challenges
These hotter 21st century drought conditions coincide with a reduction in the amount of freshwater available to coastal estuaries because of an overall negative trend in rainfall and increasing water demand for people. Meanwhile, coastal wetlands are increasingly sensitive to drought because of land use change, fragmented forests, and disconnected rivers that exacerbate drought conditions. Ultimately, drought conditions - including human water use during drought – may lead to broad-scale changes in the plant and animal communities in coastal wetlands that in turn cascade to human communities through altered recreation, fishing, and other ecosystem services.
Key Questions & Needs:
- How will drought impacts to coastal estuaries differ across Puerto Rico’s precipitation gradient?
- Long-term monitoring of estuaries across the region is needed.
- Improved understanding of the dynamics between human water use and demand (i.e. withdraws) of surface and groundwater (reduced freshwater flows) interacting with sea-level rise (increasing inshore and underground intrusion of saltwater) and the implications of short- and long-term drought on the salinity gradient and subsequent predictions of wetland habitat changes. We know that extraction of freshwater from coastal aquifers is increasing salt water intrusion. What we understand less is how the surface flows interact with this and subsequent effects on the estuary wetlands.
- How do aging infrastructure, e.g. dams in the coastal and lowland areas, influence the fresh- and saltwater balance in coastal and lowland rivers and wetlands?
References Cited
Benstead, J.P., March, J.G., Pringle, C.M., and Scatena, F.N., 1999, Effects of a low-head dam and water abstraction on migratory tropical stream biota: Ecological Applications, v.9, 656-668. Access here.
Laboy, E.N., Capella, J., Robles, P.O., Gonzalez, C.M., 2002, Jobos Bay Estuarine Profile—A National Estuarine Research Reserve: National Oceanic and Atmospheric Administration, 109 p. Access here.
Smith, K.L., Corujo Flores, I., Pringle, C.M., 2008, A comparison of current and historical fish assemblages in a Caribbean island estuary—conservation value of historical data: Aquatic Conservation: Marine and Freshwater Ecosystems, v.18, 993-1004. Access here.