Patterns in primary production of an epilithic algal community At Davies Reef, Great Barrier Reef

This study was carried out at Davies Reef in the central Great Barrier Reef region, and measured for algal biomass and primary productivity and compared to surrounding epilithic algal communities. At Davies Reef the distribution and comparative cover of damselfish territories were examined in detail.Three automatic data-logging respirometers were deployed for 24 h in the field to measure productivity parameters - light (as irradiance, Li-cor), temperature and oxygen concentration at intervals of 1 to 255 min. Primary productivity was estimated from diel patterns in oxygen flux using data from: photosynthesis-irradiance relation (Ik, Icomp and alpha); rates of photosynthesis (P) and respiration (R); primary productivity expressed as the area1 gross (Pg) and net (Pna) rates and as the net turnover (Pnb) of community biomass (B, g C m-¹). These results were compared to the parameters: damselfish species (Plectroglyphididon lacrymatus, Stegastes apicalis, S.fasciolatus, S. nigricans); reef location; date; inside/outside fish territories.At Davies Reef two experiments examined damselfish territoriality impacts on algae in detail. Algal productivity inside and adjacent to the territories of 2 species was measured using square blocks (8 x 8 x 2 cm) of Porites which had been attached to the respective reef substrata for a period of 12 months prior. In the second experiment, coral blocks with established algal covering from non-territory substrata were caged with wire mesh to exclude all large grazers for 30 d, then the productivity of the increased algal biomass on these blocks was measured by respirometry, uncaged blocks were measured at the same time. The surface area of Acropora sp. branches was calculated. Algal biomass was determined by drying coral rock samples at 60°C and scraping the surface to a depth of 2 mm. Scrapings were ground, redried, and then analysed for total organic carbon.Four major substratum categories were surveyed: fish territories with dense mats of turf algae; closely-cropped filamentous and encrusting coralline algae; bare sand; organisms other than algae.Fourteen sites were established along each of three sections of Davies Reef (northern, central and southern): front reef slope at depths of 10 (#l) and 5 m (#2); reef crest (#3); reef flat at 20 (#4), 60 (#5), 80 (#6), 110 (#7) and 145 m (#8) behind the crest; reef flat at lagoon edge (#9, 190 m from the crest); lagoon slope at 5 m (#10) and 10 m (#11) depths; back reef flat (#12); and back reef slope at 5 m (#13) and 10 m (#14) depths. At each of these sites, a 100 m tape transect was surveyed perpendicular to the section lines and the length of intercept to the nearest 1 cm for each major substratum category and damselfish territory was recorded.The aims of this study were to: To quantify differences in reef epilithic algal community metabolism. To assess the significance of these in the context of total reef primary productivity.Epilithic algal communities include coralline algae, turf algae (filamentous forms and thick turfs), corallines mixed with short turf.

本研究于大堡礁中部区域的戴维斯礁（Davies Reef）开展，测定了藻类生物量与初级生产力，并与周边的石面附生藻类群落（epilithic algal communities）进行对比。研究在戴维斯礁详细考察了雀鲷领地的分布与相对盖度。

三台自动数据记录式呼吸计（data-logging respirometers）在野外部署24小时，以1至255分钟的间隔测定生产力相关参数：光照（以辐照度计，Li-cor）、温度与氧气浓度。初级生产力通过氧气通量的日变化模式估算，所用数据涵盖：光合-辐照度关系（包含$I_k$、$I_{comp}$与$alpha$）、光合速率（$P$）与呼吸速率（$R$）；以单位面积总初级生产力（$P_g$）、净初级生产力（$P_{na}$）以及群落生物量（$B$，单位为$mathrm{g Ccdot m^{-1}}$）的净更新率（$P_{nb}$）表示的初级生产力。上述结果将与以下参数进行对比：雀鲷物种（斑带固齿雀鲷*Plectroglyphidodon lacrymatus*、顶纹密雀鲷*Stegastes apicalis*、带纹密雀鲷*S. fasciolatus*、黑密雀鲷*S. nigricans*）、礁体位置、采样日期、鱼领地内外。

在戴维斯礁开展了两项实验，以详细探究雀鲷领地行为对藻类的影响。第一项实验中，使用在对应礁体基质上附着12个月的滨珊瑚（*Porites*）方块（规格8×8×2 cm），测定该方块上两种雀鲷领地内部及邻近区域的藻类生产力。第二项实验中，将取自非领地基质、已形成藻类覆盖层的珊瑚方块用铁丝网笼住，以排除所有大型植食者，持续30天；随后通过呼吸仪法测定这些方块上增殖的藻类生物量的生产力，同期测定未笼住方块的生产力。研究还计算了轴孔珊瑚（*Acropora* sp.）分枝的表面积。藻类生物量的测定方式为：将珊瑚岩样品置于60℃下烘干，刮取表层2mm深度的样品，经研磨、再次烘干后，分析总有机碳含量。

共调查了四类主要基质类型：覆盖致密藻席的雀鲷领地、被啃食修整的丝状与壳状珊瑚藻、裸露沙地、非藻类生物类群。

沿戴维斯礁的三个区段（北部、中部与南部）各设置14个采样点，具体包括：水深10m（#1）与5m（#2）的前礁坡；礁脊（#3）；礁脊后方20m（#4）、60m（#5）、80m（#6）、110m（#7）与145m（#8）水深的礁坪；距礁脊190m的潟湖边缘礁坪（#9）；水深5m（#10）与10m（#11）的潟湖坡；后礁坪（#12）；以及水深5m（#13）与10m（#14）的后礁坡。在每个采样点，沿垂直于区段走向的方向布设100m长的样带，记录每类主要基质与雀鲷领地的截距长度，精度至1cm。

本研究的目标为：
1. 量化礁体石面附生藻类群落代谢的差异；
2. 在整个礁体总初级生产力的背景下评估上述差异的生态学意义。
石面附生藻类群落包含珊瑚藻、藻席藻类（turf algae，丝状藻类与厚层藻席）以及混合短藻席的珊瑚藻。