Benthic Mapping Pilot Project: Golden Gate National Recreation Area (GOGA)

Greene, H.; Williams, T.; Edwards, B.; Endris, C.; Ryan, H.; Niven, E.; Phillips, E.; Barnard, P.; Harmsen, F.

The objective of this project was to compile available regional seafloor mapping data for San Francisco Bay and to interpret newly collected data provided through the California State Mapping Program (CSMP) and the United States Geological Survey (USGS) to construct benthic habitat and geologic maps of the submerged lands of the Golden Gate National Recreation Area (GOGA or GGNRA). The habitat maps were constructed using the same habitat classification scheme and mapping code presently being used for the development of the CSMP habitat maps (Greene et al., 1999, 2005, 2007a). The maps comprise an integration of previous seafloor maps such as the San Francisco Bay anthropogenic habitat disturbance maps (Greene et al., 2007b).This is a pilot project that will guide future habitat characterization projects at other NPS ocean and coastal parks and serves as a working template for further mapping efforts in the GGNRA and the development of other derivative or thematic maps that may evolve from this project. The result of work by the Moss Landing Marine Laboratories (MLML)/Center for Habitat Studies/NPS/USGS is independently useful. The data obtained from this phase, however, may be used for future phases, subject to satisfaction of the data, desirability for additional data, and available funding.High-resolution multibeam sonar data in the form of bathymetric depth grids (seafloor digital elevation models, referred to as the "bathymetry") were the primary data used to interpret potential habitat types. Shaded relief imagery ("hillshade") allows for visualization of the terrain and interpretation of submarine landforms. Based on these hillshades, areas of rock were identified by their often sharply defined edges and high relative relief; these may be contiguous outcrops, isolated portions of outcrop protruding through sediment cover (pinnacles and rocks), or isolated boulders. Although these types of features can be confidently characterized as exposed rock, it is not uncommon to find areas within or around a rocky feature that appears to be covered by a thin veneer of sediment. These areas are identified as "mixed" induration, containing both rock and sediment. Broad areas of the seafloor lacking sharp and angular characteristics are considered to be sediment. Sedimentary features may contain erosional or depositional characteristics recognizable in the bathymetry, such as dynamic bedforms (dunes or sand waves). General morphologic features such as scours, mounds, and depressions were also identified using the hillshade relief imagery. Fifty-two potential benthic habitat types were defined from the interpreted data sets using the most recent version of the deep-water marine benthic habitat characterization mapping code developed by Greene et al. (1999, 2005, 2007a) and adapted specifically for estuary conditions (see Appendix I). Potential habitats ranged from hard bedrock outcrops on island and mainland flanks and some Bay floor regions, to soft, very dynamic bedforms consisting of sediment waves and ripples. Soft sediment ranged from mud and sand to bimodal (two or more grain sizes) sediment of gravel, pebbles, and cobbles. In addition, considerable anthropogenic features (i.e., pipelines, bridge abutments, dredged channels, dump sites) were distinguished. The potential habitat types are mapped within two megahabitat types after Greene et al. (2007a) – Estuary (E) and continental Shelf (S).

Benthic Mapping Pilot Project: Golden Gate National Recreation Area (GOGA)

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