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Studies of bottom-dwelling invertebrate populations on hard bottom reefs of the South Atlantic Bight

Dr. Daniel Gleason
Georgia Southern University

Our goal on this cruise is to make substantial progress on the following three projects being conducted currently by our group: 

Sea star from Gray's Reef. (Photo: Gray's Reef National Marine Sanctuary)

• We will continue to fill in as many gaps as possible by targeting species where we lack either photographs or specimens.
• We will continue collection and in situ photography of benthic macroinvertebrates that are not presently represented in our collection to expand the breadth of the web-based guide.
• We will continue meiofaunal sampling in hard bottom areas by suspending buffer pads slightly above the reef substrate. Meiofauna animals are tiny ranging in size between 0.1mm and 1.0mm.  They are typically found between grains of sediment.  Nevertheless, they are a very important component of a reef ecosystem.
• We will collect and redeploy PVC traps for assessments of cryptic fish and invertebrate marine life. We have 10 traps on 3 different reefs that we collect at periodic intervals.  Cryptic fish are especially difficult to sample because they live in small crevices in the reef and tend to hide from scientists.
• We will explore at least one new site, looking for new benthic organisms for the collection.

The specific goals of this project are to follow recruitment of benthic marine invertebrates to newly established patches of bare reef off the coast of Georgia and document changes in patch community structure over time.  In addition, we hope to determine if recruitment of benthic marine invertebrates to hard bottom sites off the coast of Georgia is primarily from local or long distance dispersal of larvae. This project has practical implications for management of the GRNMS because it will provide baseline information that can be used to assess what short and long-term impacts might result from user-induced damage to the bottom. This project was initiated during the May 22-30, 2004, Nancy Foster cruise on J-Reef and has been expanded to a site within the boundaries of Gray’s Reef National Marine Sanctuary.  Activities conducted for this project will consist of the following:

• At J-Reef we will continue photographic monitoring of the 20 natural plots (30x30 cm each) established and 10 pieces of artificial substrate deployed in summer 2004.
• At the GRNMS monitoring site we will re-establish treatment plots originally set up on the 2005 Nancy Foster Cruise by again scraping clean 30 x 30 cm sections of substrate on the top of the ledge using a chisel and hammer. There will be a total of 10 haphazardly selected replicate plots that will be scraped clean and 10 unmanipulated controls.
• At J-Reef, we will stain with alizarin red-S colonies of Oculina arbuscula that have settled on paving tiles deployed in 2005. The purpose of this staining will be to provide a mark on the skeleton that can be used for future measures of coral growth.
• We will use the multi-beam capabilities of the Nancy Foster to map hard bottom areas at the following sites: Gray’s Reef National Marine Sanctuary Monitoring Site and Patch Reef #1, J Reef, the R2 Tower hard bottom, and Ledge #6.

The goal of this project is to determine if established communities of sessile (or sedentary) benthic invertebrates on individual hard bottom reefs vary both spatially and in time and to determine if sediment is a major factor structuring these communities.  Preliminary observations indicate that sessile benthic invertebrate populations on South Atlantic Bight reefs vary over small spatial scales (i.e., a few meters) on individual outcrops and in time on scales that range from seasonal to annual. Thus, monitoring of benthic invertebrate populations in this system must be conducted over time scales that allow seasonal variation to be differentiated from longer term patterns or stochastic events.  In the absence of such monitoring, our ability to predict trends and to manage the system is compromised. This is a new research direction and will be initiated during the cruise as follows:

• Monitoring of sessile benthic invertebrates will be initiated by quantifying populations in 0.5 x 0.5 m (0.25 m²) quadrats placed adjacent to a 30 m long transect line. A total of 3 transect lines will be established at each of 2 sites (J-Reef and GRNMS Monitoring Site): one following the ledge immediately adjacent to the scarp (ledge face), one placed 2 m away from the scarp on the sandy ledge, and one placed 5 m away from the scarp on the sandy ledge. Transect lines will be placed in these positions to take into account gradations in community composition that appear to occur on these ledges.
• Sediment traps will be deployed along the same transect lines used to monitor temporal and spatial variation in the benthic invertebrate communities. These traps will consist of 5 cm diameter x 30 cm length polyvinyl chloride pipe capped at one end. A total of 30 traps, 10 replicates along each of the 3 transect lines at each site, will be attached with plastic cable ties to the stainless rods that mark the transect line positions. This will ensure that sedimentation rates are quantified within close proximity to the monitoring of benthic cover. All traps along a single transect line will be separated by a horizontal distance of no less than 2 m.