Resource Description: NODC Accession Number 9900151
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Mamala Bay Study MB-9 Department of Oceanography University of Hawaii
Study of recruitment patterns of marine benthic invertebrates in Mamala Bay. It was a process-oriented measure of ecosystem response to pollution.
SITE Latitude Longitude Depth (deg,min) (deg,min) (m) ----- ------------------- ----------------- -------- 1 21 16.808 N 157 58.754 W 72 2 21 16.770 N 157 54.333 W 72 3 21 14.988 N 157 49.744 W 72 4 21 17.471 N 157 58.952 W 16 5 21 17.908 N 157 57.090 W 16 6 21 17.611 N 157 55.032 W 16 7 21 17.317 N 157 52.035 W 16 8 21 16.411 N 157 50.682 W 16 9 21 15.146 N 157 49.405 W 16
METHODOLOGY Larval recruitment and distribution patterns were evaluated at moorings sites chosen according to three criteria. First, sites of maximal point-source influence (Sand Island outfall), maximal non-point source influence (off Pearl Harbor) and minimal pollution influence (off Diamond Head) were targeted along a single depth contour. These mooring sites allowed direct comparisons of larval availability and recruitment in point source, non-point source, and control areas; bottom depth at these sites (termed "deep sites") was set by the depth of the Sand-Island outfall diffuser (72 m). Second, areas near sources of non-point pollution ( Pearl Harbor, Keehi Lagoon, Honolulu Harbor and the Ala Wai Harbor) and an area with minimal pollution influence (off Diamond Head) were targeted along a depth contour (16 m) where recruitment rates should be representative of subtidal fauna. These sites (termed "shallow sites") allowed comparisons of larval availability and recruitment between areas presumed to be influenced by major non-point sources of pollution in Mamala Bay, and a control area. As a third criterion, settlement moorings were located within close enough range to Honolulu Harbor to allow diver sampling of all nine sites in a single day. To ensure environmental similarity across sites, controlled settlement substrates (ceramic plates) and larval traps were placed on midwater moorings. We chose not to conduct recruitment studies on the seafloor to avoid biases due to dramatically varying flow regimes (resulting from differences in seafloor topography), varying bottom types, and varying rates of demersal fish predation on larvae and recruits between sites. Recruitment plates and larval traps were placed at two water depths at deep sites; 15 meters (for spatial comparisons with shallow sites), and 35 meters (where plume models suggest that sewage concentrations from the Sand Island outfall are often maximal; Noda, pers. comm.). Recruitment plates and larval traps were placed at 10 m at shallow sites. Ideally, upper sampling depths would have been identical at shallow and deep sites. However, Coast Guard rules prevented subsurface moorings from being shallower than 15 meters in water depths exceeding 18 meters, and seafloor proximity precluded placing plates and traps deeper than 10 m at shallow (16-m bottom depth) sites. Larval recruitment rates were quantified using ceramic tiles as settlement substrates (dimensions, 20 x 9.5 x 1cm). Two to four replicate tiles, held in specially designed and fabricated aluminum frames (Figure 3.2), were used on each settlement mooring. Clean tiles were placed on frames by divers, allowed to collect recruits for 1-3 wk, and then recovered by carefully sealing each plate (while still underwater) in a separate tupperware container. In the lab, tiles were gently washed with tapwater over a 250 micrometer sieve. Animals remaining on tiles or retained in the sieve were then identified and enumerated under a dissecting microscope. Larval availability was quantified using two different methods. During initial stages of the study, larvae were sampled by conducting plankton tows at shallow stations, as initially proposed. Plankton tows were conducted with a 0.5 meter diameter, 202 micron Nytex) net. A General Oceanics flowmeter was rigged to the front of the net to quantify tow volumes. Two five-minute replicate tows were taken at stations 5, 6, 8 and 9. Plankton tow samples were immediately fixed in a 10% formalin solution, transferred in the laboratory to 80% ETOH, and then sorted under a dissecting microscope. Several months into the study, it became clear that larval numbers in plankton tows did not correlate well with recruitment at any mooring sites. A likely explanation was that plankton tows quantify larval concentrations at a particular point in time, but not the time-integrated flux of larvae past a stationary point; in contrast, recruitment tiles record larval availability or flux throughout their 1-3 wk deployment period literature that sediment traps with aspect ratios 10 record larval flux in flow regimes typical of shallow coastal waters. Therefore, after becoming aware of these new methods for measuring larval flux, we elected to evaluate larval availability at mooring sites using time-integrative, larval traps. Upward-facing plastic tubes (i.e., larval traps) with a diameter = 5 cm, and a height = 60 cm, were deployed 25 cm from the sides of moorings to sample the horizontal flux of larvae past stations 4 through 9. The bottom 10 cm of larval traps contained a 10% formalin solution for larval fixation. NaCl (100 ppt) was added to the formalin solution to form a high-density layer that generally remained undisturbed in the bottom of the traps throughout deployment and recovery. This solution was dyed with Rhodamine WT for sample-integrity verification. Recovered traps that did not contain an intact layer of dyed formalin solution were not analyzed. Two replicate traps were deployed at each station synchronously with deployment of settlement plates. After recovery, larval trap samples were transferred to 80% ETOH and larvae were identified and enumerated under a dissecting microscope.
Smith, Craig R. and Parnell, Ed P., 1995. Recruitment Patterns of Marine Benthic
Invertebrates in Mamala Bay: A Process-Oriented Measure of Ecosystem Response
to Pollution. Mamala Bay Study, Project MB-9. Mamala Bay Study Commission. 1996.
Mamala Bay Study Final Report.
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