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PRD 00/01-07 Monitoring Coral Reef Macroalgae

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PRD 00/01-07 Monitoring Coral Reef Macroalgae PRD 00/01-07 Monitoring coral reef macroalgae: different pictures from different methods 1 2 2 M.W. Miller , RB. Aronson , T.J.T.Murdoch 1) NOAA-Fisheries, Southeast Science Center, 75 Virginia Beach Dr., Miami FL, 33149 USA phone: 305-361-4561, FAX: 305-361-4562, emai1: margaret.w.miller(2linoaa.gov 2) Dauphjn Island Sea Lab, 101 Bienville Blvd., Dauphin Island, AL 36528 USA; and Department of Marine Sciences, University of South Alabama, Mobile, AL 36688 USA Running Head: Monitoring reef macroalgae Keywords: macro algae, coral reef monitoring, video, biomass ABSTRACT Despite the low coral abundance and high macroalgal abundance on many coral reefs worldwide, coral reef monitoring programs often focus narrowly on hard corals. As part ofa benthic monitoring program to assess the effects of no-take marine reserves in the Florida Keys National Marine Sanctuary, we comptired two measures of macroalgal abundance: percent cover estimated from video trunsects and biomass estimated from harvested quadrats. The overall patterns of macroalgal abundance were sirnilar between the two methods, but the species-specific resolution of assemblage structure was much greater in the biomass sampling. Neither method provided adequate estimates of cristose coralline algae nor algal turf abundance. Considering the present emphasis Gn improving and coordinating coral reef monitoring on both national and global scales, we advocate inclusion and standardization of the estimation and reporting of reef macroalgal abundance. 2 INTRODUC'nON Monitoring is receiving increased emphasis as a means to improve the information on status and trends required for adaptive management and conservation of coral reefs. Examples include the activities of the US Coral Reef Task Force (USCRTF 2000, hLtp://coralreefgov/WG-reg2rts.htrni) and the 1999 International Conference on Scientific Aspects of Coral Reef Monitoring, Assessment, and Restoration (h4p://www.nova.edu/ocean/ncri/conf99. While it is well-recognized. that proliferation of macroaigae is one of the major results of reef degradation (Ginsburg 1994, Wilkinson 2000), coral reef monitoring methods and ieporting almost always focus on corals. IvIacroalgae are counted incidentally andiumped, often into a single category xmiled "algae". Such lumping obscures important fimetional groupings (Steneck and Diether 1994) that could elucidate reef status and fimiction. As part of the benthic monitoring program of the no-take zones in the Florida Keys National Marine Sanctuary (FKNMS), we compared results of a standard reef monitoring method (video taping transects) with a macroalgal-focased method (harvesting and quant&f* biomass) to =mine the differences in macroalgal abundance and assemblage composition measured by the two methods. METHODS SWdy sites - Permanent sites were established in selected no-take zones and at nearby reference sites in October 1997. Study sites were established on six reefs within two depth ranges, 7.5-10.5 rn (25-35 ft) and 13.5-17.4 m (45-58 ft). Table I lists the sites and their characteristics. The sites were permanently marked and their DGPS coordinates were recorded. 3 Video transects - At each of the six fiked sites, ten haphazardly located transects were videotaped during May 1998 and again in May 1999. Each transect consisted of a 25 in x 0.4 in area. A reference arm was mounted on the camera housing to maintain a constant distance between the video camera and the substrate to standardize the size of the field of view at 0.4 ra width. The video transects were analyzed according to welt-established protocols (Aronson et al. 1994; Aronson and Swanson 1997). Automated procedures were developed for video fi-ame capture using digital video equipment and commercially available hardware and software. Each transect was characterized with 50 non-overlapping frames. The captured images were analyzed using point-counting procedures (10 points per frame), and the frames and data were archived on CD-ROK Quality assurance/quality control procedures were instituted, consisting of multiple, trained individuals doing the same point counts. These QA/QC assessments indicated that the , data are of high quality, with <1% error in the assignment of points to the following algal genera identifiable from the video: Dictywa, Lobophora, Stypopodium, Halimeda, Sargassum, and a category of "other macroalgae". Macroalgal biomass - To quantify macroalgal biomass, a 40 x 40 cm quadrat was placed at random intervals along 25-m transects videotaped at the same time in Afay 1998. In 1999, the biomass samples were collected in June, approximately I mo following the video transect sampling. The quadrats were placed systematically around the permanent marker buoy at each site, at intervals of 5 fin-kicks along a long-shore heading. The 40-cm size of the quadrat was chosen to match the 40-cm path of the vkleo sample. All of the seaweed biomass that one could 4 grasp, pluck, and deliver into a sealed plastic bag in a 5-min span was harvested from each quadrat. Back on the boat, each plastic bag was careffilly drained. The samples were placed in larger, labeled plastic bags and frozen for storage. In the laboratory, each quadrat sample was thawed, placed in a sieve and rinsed with tap water to remove sediment, and then rinsed from the sieve into a sorting tray. Samples were sorted to genus into pre-weighed aluminum weighing dishes, dried for at least 24 hr at 65-700C, and weighed. Data Apaiysis - Analysis of Ilariance (or non-parametric Y^ruskal-Wallis ANOVA, incasesof violation of theassumptions of parametric statistics) was used io test for significant variation among sites, as coral reef monitoring programs are often designed to do. Separate ANOVAs were performed for each parameter, macroaigal cover and macroalgal biomass, for each year, and for the deep and shallow depth strata. Significant parametric (or Kruskal-Wallis) ANOVAs were followed by Tukey's (or Dunn's) aposteriori pairwise comparisons. RESULTS Macroalgal Abundance - In most cases, the pattern of relative macroalgal abundance among sites was similar when assessed as either cover or bio s (Fig. 1). For example, the deep sites in 1999 showed high macroalgal abundance at South Carydort and at Pelican Shoal and low abundance at the other sites, whether abundance was determined by video sampling for percent cover or by harvesting for bioma s (Fig 1). Although all 12 ANOVAs indicated sigifficant variation among sites, there were no consistent differences in macroalgal abundance between the 5 no-take reserves and the reference sites. Macmalgal Cominunity Structure - While the video and biomass sampling methods showed consistent patterns in total macroalgal abundance, they provided sharply contrasting characterizations ofmacroalgal assemblage structure. The video transect sampling showed that the sampled reefs were strongly dominated by Dictyota spp. Almost half the video samples (I I out of 24 depth/year/site combinations) showed over 501/o of benthic cover to be Dictyota. All the video samples indicated that Diclyota comprised more than 78% of the macroalgal assemblage, and for most samples it was over 90% (Fig. 2). Four other macroalgal genera were disc2mable in the video sampling, but these included no red algal genera. The biomass sampling yielded a more balanced picture of the macroalgal assemblage. The green calcareous Halimeda spp. were strongly represented at levels up to -40% of the macroalgal biomass. Numerous other genera were well-represented in the biomass sampling (Table 2) and represented up to 871/o of the macroaigae in a given sample (Fig. 2, PS Shallow 1999). Twenty- four genera were identified in the biomass sampling. DISCUSSION While video sampling and harvested biom s sampling yielded smi pictures of overall macroalgal abundance, there were sharp differences in the representation of macroalgal diversity and assemblage structure. The recommended method depends on the particular question one is trying to answer. In most coral reef monitoring programs, the primary questions are: 1) what is the status or "healff'of the monitored coral reef? and 2) is it changing over time? While patterns can be discerned in monitoring studies, questions of causation can be only partially answered. 6 Although the proliferation of macroalgal standing stock is widely recognized as a manifestation of coral reef decline, the fact is that most major regional coral reef benthic monitoring programs report solely on hard coral abundance (e.g., Haw* htti2:/Icr=.wcc.hawaii.edu/Overview/ or Brown et al. 1999; Great Barrier Ree^ Australia, httv..-//www.airns 9ov.au/i3aLres/research/reef-monito reef-monitorin a-index. or Sweatman. 1999; the Red Sea, Loya et al. 1999). In some cases, extensive effort has been expended to devise a robust, powerfW, and reasonably priced sampling scheme for corals (e.g., Brown et al 1999). However, no mention is made of any analysis nor results fi)r macroalgae despite often explicit intent to detect ^-.hanges related to impoi tant reef threats ^acluding invasive algal species and algal outbreaksp.wcc.ha,,vaii.edu/Overviel,v/3. (e.g., ho://crarn P ethodsl). V Many of these large-scale, integrated monitoring programs utilize video transect sampling. It would appear that resource limitation simply precludes the extraction and/or reporting of macroalgal data. Biomass harvesting, while destructive, yields much better taxonomic resolution than video sampling and can provide insights into the fimction of reef ecosystems. There is ongoing controversy in
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