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Products from an Ecological Characterization of the NPS Salt River Bay National Historical Park and Ecological Preserve, U.S. Virgin Islands


Salt River Bay National Historical Park and Ecological Preserve (hereafter, SARI or the park) was created in 1992 to preserve, protect, and interpret nationally significant natural, historical, and cultural resources (United States Congress 1992). The diverse ecosystem within it includes a large mangrove forest, a submarine canyon, coral reefs, seagrass beds, coastal forests, and many other natural and developed landscape elements. These ecosystem components are, in turn, utilized by a great diversity of flora and fauna. A comprehensive spatial inventory of these ecosystems is required for successful management. To meet this need, the National Oceanic and Atmospheric Administration (NOAA) Biogeography Program, in consultation with the National Park Service (NPS) and the Government of the Virgin Islands Department of Planning and Natural Resources (VIDPNR), conducted an ecological characterization. The characterization consists of three complementary components: a text report, digital habitat maps, and a collection of historical aerial photographs. This ecological characterization provides managers with a suite of tools that, when coupled with the excellent pre-existing body of work on SARI resources, enables improved research and monitoring activities within the park (see Data for a list of data products).

A collection 184 color, black and white, black and white infrared, and color infrared aerial photographs of the Salt River area from the 1970's to 2000 were obtained from several federal agencies for this assessment (individual photos are available from NPS). Photographs from selected years were digitally oriented in geographic space (orthorectified) and then used to create several habitat maps of the park (mosaics are available in the Data section of this DVD). The most current photographs, from year 2000, were used to create a map of fifty terrestrial and marine habitat types visible in the imagery. This map covers the entire land (145 hectares) and mangrove area (19 hectares) within the park, all of the benthic habitat within Triton, Sugar, and Salt River Bays , and much of the offshore benthic habitats (250 hectares). This map, created with a minimum feature size of 10 by 10 meters, is the first detailed spatial characterization of the SARI ecosystem. The time series of photographs were used to create maps of changes to seagrass and mangrove distributions that have occurred over the last three decades. This group of maps and images were used to frame the discussion of each major habitat type, faunal group, or environmental category in the text report for this ecological characterization.

The text portion of the report is divided into sections based on physical characteristics (e.g. geology, water quality, currents), habitat types (e.g. land cover, coral reefs, mangroves), and major faunal groups (e.g. fish, birds). Each section includes an overview, methods, results, and a discussion of linkages with other components of the SARI ecosystem and surrounding environment.

Physical characteristics of the SARI area described in this report include currents, climate, water quality, geology, and bathymetry. Water currents within the park are primarily wind and tidal driven. These play an important role in the transport of sediments along the shelf and canyon axis and consequently are a major control on reef characteristics of the canyon walls. The climate of the park is controlled primarily by the seasonal changes associated with the trade winds which are interrupted by weak cold fronts in winter and hurricanes in the summer and fall. Water quality within the bays is usually within acceptable values for Class B waters although dissolved oxygen, turbidity, and bacterial load of some sites farthest from the bay mouth are periodically in violation of allowable pollutant levels. The geology of the region has been well characterized including the underlying terrestrial and marine formations as well as the sediment accretion, erosion, and transport patterns of the bays and shelf. Bathymetry within the park has changed considerably over the last fifty years due to dredging activities and will likely continue to change at an accelerated rate relative to natural conditions due to development and erosion in the watershed.

Reef and hard bottom habitats in the canyon were once among the best studied and characterized coral structures in the world at the time the NOAA National Undersea Research Program (NURP) saturation diving facility was in operation at the site. Since the closing of this facility following Hurricane Hugo in 1989 this is no longer the case. Reefs within SARI but outside of the canyon have received virtually no attention. Based on the year 2000 maps, total two-dimensional coral reef and hard bottom area within SARI covered 116.3 hectares, with over 41 species of coral documented in the canyon from existing literature.

Seagrass and algae communities within the canyon were also once studied intensively using the NOAA/NURP diving facility but have since gone largely unmonitored. Our maps indicate that seagrass distributions within the bays declined by ~13% overall from the 1970's to 1992 with the greatest apparent change occurring in the northwestern portion of Salt River Bay. By 2000, seagrass area had changed little from its 1992 extent.

The mangrove forests of the park were once among the most impressive in the region although they have undergone perhaps the most dramatic changes of any SARI ecosystem component in the last 30 years. Hurricane Hugo in 1989 killed over half of the 1988 mangrove stand, approximately 12 hectares of forest, and reduced the density of much of the remaining canopy. Despite this catastrophic loss, forests are recovering both naturally and with human assistance.

Notable evidence of past and recent human alteration, including dredging and construction, are noted in the grey literature, and are also visible in aerial photographs. Despite this and the expansive residentially zoned development areas within the park, forests dominate current (2000) terrestrial land cover, accounting for 106 hectares.

Fish communities within SARI are quite diverse considering the park's relatively small area. The presence of mangrove, seagrass, reef, bay, shelf edge, and access to offshore habitat within park boundaries all contribute to the high diversity of fish. Recent studies have noted 57 species in mangrove habitats, and nearly 200 on the walls of the canyon.

The bird fauna utilizing SARI are similarly diverse. The SARI area contains an array of potential avifauna habitat, including sandy beaches, mangrove stands, and mud flats. The most recent bird census data were collected by the Virgin Islands Division of Fish and Wildlife, but were not available during the preparation of this report.

The aerial imagery, habitat maps, and text report that make up this assessment are complimentary, together providing research, monitoring, and management tools for the park. The images can be used to map additional ground features, document historical changes, and serve as a baseline against which future imagery may be compared. Habitat and land cover maps will assist with the design of monitoring schemes, selection of research sites, and identification of potential habitat for species of interest. The discussion and analyses contained in the text highlight established knowledge, explore spatial aspects of prior research, and identify information gaps and threats that may guide future monitoring and research. Together, these components provide a variety of information that will facilitate the current and future stewardship of the diverse resources contained within SARI.

For more information:

Additional information in the following subject areas is available from the web sites listed below. ...for information on the National Park Service on St.Croix, general information on SARI, hard copy, and scans of aerial photographs of SARI, and all other data products in this report.
Written requests for information on other inquiries should be sent to...

2100 Church Street #100
Christiansted VI 00820-4611
or by phone at 340-773-1460
or fax at 340-773-2950
or by email at …for information on benthic and land cover maps in this report …for information on NOAA aerial photography …for information on NOAA’s Coral Reef Early Warning System …for information on the local government involved in research and management at SARI. The site includes links to information on such topics as water quality and wildlife monitoring in the U.S. Virgin Islands. …for information on Comprehensive bibliography of Salt River research