David Klumpp

Algal food supply and grazer demand in a very productive coral-reef zone

Abstract Benthic algal-community net primary productivity (NPP) and herbivore ingestion rates were quantified for the productive zone dominated by the aggressive surgeonfish Acanthurus lineatus (Linnaeus) on the windward crest of Davies Reef, Great Barrier Reef, northeastern Australia. The aim was to evaluate whether or not food supply could be a limiting factor for grazers. Grazing accounted for 65°o of NPP in carbon terms during summer, and 43°o in winter. The most important grazers were the resident territorial surgeonfish Acanthurus lineatus (41°o of NPP in summer) and damselfish Stegastes apicalis (De Vis) (15°o). Grazing rates increased 1.8-times, while NPP increased only 1.2-times, between winter (mean ambient temperatures 23°C) and summer (28°C), but in both summer and winter grazing was significantly less than NPP. NPP which was not accounted for by grazing may have been lost through secretion of dissolved organic matter by algae, export of algal fragments from the zone and decomposition in situ, processes which have been little studied.

The role of epiphytic periphyton and macroinvertebrate grazers in the trophic flux of a tropical seagrass community

Abstract Biomass and production of epiphytic periphyton, and the abundance, distribution and grazing rate of epifauna were measured in tropical seagrass beds in the Philippines. Periphyton comprised mainly detritus, diatoms and filamentous algae (Polysiphonia sp. 1, Centroceras clavulatum (C. Agardh) Montagne, Ceramium gracillimum Harvey and Cladophora sp.). Mean biomass of periphyton was 0.16 mg ash-free dry weight (AFDW) cm−2 frond of Enhalus acoroides (L.f.) Royle and 0.24 mg AFDW cm−2 frond of Cymodocea serrulata (R. Br.) Aschers. and Magnus. Total periphyton biomass per unit area (m2) of seagrass bed varied between habitats because of differing densities of seagrass, and ranged from 598 to 1061 mg AFDW or (24–646 mg C). Maximum (midday, summer) in situ rates of photosynthesis and respiration by epiphytes colonising artificial seagrass material averaged 11.6 μg O2 cm−2 h−1 and 2.0 μg O2 cm−2 h−1, respectively. Daily net productivity was 14 μg C cm−2 frond. Productivity of epiphytes per area of seagrass bed varied with site (36–77 mg C m−2 day−1). Relative to biomass, these data show that epiphytes are highly productive, with turnover times of 6–8 days, compared with known values of 30–100 + days for tropical seagrass fronds. The epifaunal grazer community was dominated by a few species of gastropod molluscs (especially Strombus mutabilis Swainson and Cerithium tenellum (Sowerby)). Within habitats, numbers of grazers on particular seagrass species were directly related to their available surface. Three groups of grazers were identified: those occurring on fronds day and night (e.g. S. mutabilis); those foraging over sediment during the day and fronds at night (e.g. Cerithium tenellum); those mainly confined to sediments (e.g. Strombus urceus L.). All epifaunal grazers exhibited upward movement into the seagrass canopy at night. Grazing was non-selective, removing the periphyton, except for the unutilised encrusting coralline algae, in proportion to abundance. Epifaunal grazers consumed between 20 and 62% of periphyton net production and, as in temperate systems, must therefore play a major role in the trophic flux of this tropical seagrass community.

Partitioning among grazers of food resources within damselfish territories on a coral reef

The net production of the turf-rich algal communities within the territories (mean planar area = 1.43 m2) of the herbivorous damselfish Stegastes apicalis (De Vis) on Davies Reef, Great Barrier Reef, ranged between 3.116 and 3.732g C·territory−1 ·day−1 and this was partitioned almost entirely among grazers. The most important grazers of this production were the resident damselfish (25 and 38% of net production in winter and summer, respectively), mobile cryptofauna, such as amphipods, copepods, and polychaetes (31% of production in summer), and “invading” fishes, especially scarids (14% of production in winter). Grazing rates of damselfish (mean size 63 g wet wt) increased from 773 mg C · fish−1· day−1 in winter to 1433 mg C · fish−1· day−1 in summer. This damselfish selected mainly fragile filamentous red, e.g., Centroceras clavulatum, Ceramium spp., and Coelothrix irregularis, and blue green algae, Lyngbya sp. Other grazers, e.g., blennies and macroinvertebrates (Turboand Trochus spp.), were insignificant as consumers of territory food resources because of their low biomass density. The view that damselfish territories are important in reef trophodynamic and nutrient regeneration processes is supported since their cover of the reef flat is extensive, turnover of their productive algal community by grazers is high, and grazer waste products are subsequently released locally.

Feeding ecology and trophic role of sea urchins in a tropical seagrass community

The grazing impact of urchins on seagrass and algal resources, and the relative importance of this to the lower-level trophic flux of a tropical seagrass community were investigated. Thalassia hemprichii (Ehrenb.) Aschers. accounted for 80–93% of seagrass frond biomass at Bolinao in the Philippines. Growth rate of seagrass was 6.6 mm per shoot day−1, or 2.3 mg AFDW per shoot day−1. Production of seagrass fronds per unit area of seagrass bed varied with location from 870 to 1850 mg AFDW m−2 day−1. Urchin density ranged from 0.9 to 4.2 m−2, with Tripneustes gratilla (L.) and Salmacis sphaeroides (L.) being the most common species. Tripneustes gratilla fed mostly on attached seagrass fronds (77–89% of diet), especially Thalassia hemprichii, whereas S. sphaeroides was a generalist, consuming Thalassia hemprichii fronds (13–65%), detached seagrass debris (5–39%), the red alga Amphiroa fragilissima (L.) Lamour. (0–30%), algal-coated sediment and rubble (0–51%) in proportions that varied with the availability of preferred food types. Live Thalassia hemprichii fronds were clearly preferred over macroalgae or dead seagrass fronds by Tripneustes gratilla, but S. sphaeroides consumed all three food types without preference. Both urchins avoided the common brown alga, Sargassum crassifolium J. Agardh. Urchins absorbed 73–76% of organic matter in seagrass fronds with epiphytes (75% of DW), and 55% of that in epiphyte-free fronds. Seagrass debris and the macroalgae A. fragilissima were of lower food quality as they were lower in organic matter, and this matter was absorbed less efficiently by urchins. Rates of ingestion (IR in g WW per urchin day−1) were proportional to body weight (W in g WW) according to the functions: IR = 0.56W0.34 (T. gratilla) and IR = 0.17W0.53 (Salmacis sphaeroides). Predicted grazing impact of urchins on seagrass resources varied spatially and temporally. Estimated annual grazing rate at the main study site was 158 g AFDW m−2, equivalent to 24% of annual seagrass production, but owing to large changes in urchin population structure and density, grazing impact is expected to vary from 100% at different times of year. A synthesis of knowledge on the lower-level trophic pathways in this system indicates that seagrass-urchin and periphyton-epifauna grazing interactions are both important in their contribution to overall trophic flux.

Temporal and spatial patterns in primary production of a coral-reef epilithic algal community

Photosynthesis-irradiance (P-I) relationships of the epilithic algal community (EAC) on a coral reef (Davies Reef, Great Barrier Reef, Australia) altered with season and depth in a manner that minimized the effect of reduced irradiance on winter productivity. When compared with algae from reef-flat habitats, the algal community on the reef slopes had a higher photosynthetic efficiency, equal biomass-specific productivity and half the biomass and areal productivity. Productivity and biomass of the EAC did not vary between different reef habitats at the same depth. Productivity of the EAC across all reef habitats was 1.6 times higher in summer than in winter. Crustose coralline algae, which dominate the EAC on reef crests and decrease in importance with depth and distance from the crest, displayed a high biomass, a low biomass-specific productivity and an areal productivity that equalled that of fine turf algae (< 1 mm high). The EAC dominated coverage in all reef habitats except the front-reef slope where there was a high coral cover. Productivity of the EAC per unit area of reef was highest (up to 3 g C · m −2 reef · day−1) around the shallow-reef margins due to a combination of maximum EAC coverage, an abundance of thick highly productive damselfish-turf and a high substratum relief. Productivity decreased with distance back from the reef crest until it stabilized over the rear two-thirds of the reef flat. The reef slopes at 10 m were the least productive habitat in terms of the contribution by the EAC. Comparisons with data on total reef-flat productivity indicate that the EAC is responsible for one-third of the gross production and most of the net production.

Nutrition of the giant clam Tridacna gigas (L.) I. Contribution of filter feeding and photosynthates to respiration and growth

The total carbon requirements (growth + respiration) of the host tissues of the giant clam Tridacna gigas from Davies Reef on the Great Barrier reef were measured, and compared with rates with which nutrients were acquired from the two potential sources, translocated photosynthates (estimated from oxygen flux-CZAR method) and filter feeding. Results showed that the giant clam is an efficient utilizer of particulate organic matter available in reef waters (Davies Reef mean: 97 μg C·1−1), retaining on average 75% of particles between 2 and 50 μm, and absorbing from them 54% of C. Clearance rates (CR, 1·h−1) of clams were size dependent as defined by the function: CR = 1.85 W0.58 (r2 = 0.85, n = 56). There are major differences between typical non-symbiotic bivalves and Tridacna gigas regarding the relative allocations of energy to respiration and growth. The proportion of carbon deposited in tissues relative to that respired is high in giant clams relative to completely heterotrophic bivalves. We conclude that autotrophy is the major source of carbon to this clam, potentially capable of satisfying all respiratory requirements of the host. However, the potential importance of heterotrophy to total energy needs of the host is also significant and changes with the size of clam. The spectacular rates of growth in this clam are such that filter feeding is able to provide 65% of the total carbon needed both for respiration and growth in small clams (100 mg dry tissue wt), whereas large clams (10 g) acquire only 34% of their carbon from this source.

Ecological correlates of foraging periodicity in herbivorous reef fishes of the Coral Sea

Abstract Diurnal feeding activity patterns of five herbivorous reef fishes were studied at two sites in the Coral Sea, Motupore Island and Davies Reef. A marked peak in feeding activity in the afternoon was found for the damselfishes Pomacentrus flavicauda Whitley, Stegastes apicalis (De Vis) and Plectroglyphidodon lacrymatus (Quoy et Gaimard), and the surgeonfishes Acanthurus lineatus (L.) and Ctenochaetus striatus (Quoy et Gaimard). Pomacentrus flavicauda and Ctenochaetus striatus exhibited weak territorial aggression, but afternoon foraging was as marked in them as in the other species. Pomacentus flavicauda , Stegastes apicalis, Plectroglyphidodon lacrymatus and Acanthurus lineatus fed on the algal turfs within their territories. Although some Ctenochaetus striatus on the outer reef flat foraged on algal turfs, most individuals fed over crustose-coralline-algal surfaces and rocks with diatom and sparse filamentous-algal coverage; afternoon peaks of feeding were found in both cases. Afternoon foraging was not found to be a metabolic response to diurnal temperature increase, and did not correlate directly with average tidal, wind (water movement), or light-intensity patterns. Although ambient dissolved-oxygen maxima did correspond with afternoon foraging, there was no evidence that oxygen was limiting to fish metabolism and no sign was found of an intrinsic respiratory rhythm in one species, Plectroglyphidodon lacrymatus . Assuming negligible exudation of soluble organics, measurements of community net oxygen flux by turf algae indicate that photosynthetic carbon accumulated through the daytime at both sites in the Coral Sea. At Motupore Island, organic matter tended to rise and crude-fibre levels fell diurnally in turf algae collected from the reef, demonstrating a measurable increase in more-assimilable carbon later in the day. Afternoon foraging may be an adaptation by herbivorous reef fishes to the diurnal build-up of photosynthate in productive microalgal communities, particularly turfs.

Detrital pathways in a coral reef lagoon II. Detritus deposition, benthic microbial biomass and production*

Coral reef lagoons have generally been regarded as sinks for organic matter exported from more productive reef front and reef flat zones. The object of this study was to examine the importance of detritus as a carbon source for benthic communities in the lagoon at Davies Reef, central Great Barrier Reef. We report the results of seasonal measurements, taken in 1986, of bacterial numbers and production, protozoan numbers, community primary production and respiration in the sediments of Davies Reef lagoon. Deposition rates of organic matter in the lagoon were also measured. Deposition rates (±1 SE) of carbon ranged from 9.2 (±1.5) to 140.7 (±10.3) mg Cm-2d-1. Deposition rates were highest in winter and spring, lowest in summer. Rates of bacterial production ranged from 4.7 (±0.2) pmol thymidine incorporated g-1 dry wt (DW) h-1 in winter to 23.5 (±1.0) pmol thymidine incorporated g-1 DW h-1 in spring. The number of ciliates ranged from 65 (±10) to 356 (±50) cm-3 through the year and the number of large (≥20 μm) flagellates from 38 (±7) to 108 (±16) cm-3. There were no clear relationships between the sediment organic content, detrital input or temperature and the rates of bacterial processes, community metabolism or the standing stocks of microbes in the lagoon. The relative significance of detritus and in situ primary production as sources of carbon in the lagoon varied with season. In summer and autumn, detritus was less important than primary production as a source of carbon (4 to 27% of total carbon input). In winter and spring, detritus input became more significant in supply of carbon to the sediments (32 to 67% of the total carbon input). The lagoon does not simply act as a sink for carbon exported from the reef flat. We calculate that only 5% of the net reef flat primary production reached lagoon sediments in summer, but nearly 40% in winter.

Biological effects of pollutants in Australian tropical coastal waters: Embryonic malformations and chromosomal aberrations in developing fish eggs

Abstract This study establishes baseline information on the biological effects of contaminants in the tropical marine environment of Australia, using as indicators the incidence of embryonic malformation and chromosomal aberration in developing fish eggs. Eighteen sites were examined, including nearshore coastal waters with varying degrees of human activity, and offshore locations on the Great Barrier Reef, in order to assess whether there is any present indication of contaminant effects in the region. Natural background levels of abnormality in embryos and chromosomes of fish are both expected to be less than 10%. All sites examined had chromosomal aberrations well within this limit, and as such, could be classified as uncontaminated by toxicants which have mutagenic effects. Mean rates of embryonic malformation varied with site location, and ranged between 6 and 24%. Coastal sites adjacent to areas used for intensive farming, or with major urban centres, had consistently higher levels of embryo deformity than sites on the Great Barrier Reef. Thus, whilst the incidence of embryonic malformation for the region as a whole (mean 12%) was low in comparison with that of polluted waters (30–80% reported), there is some concern that human activities may be having a localized effect in the region.

The use of transplanted cultured tropical oysters (Saccostrea commercialis) to monitor Cd levels in North Queensland coastal waters (Australia).

Bivalves are commonly used to detect metal pollution in the marine environment. Commercially cultured Milky oysters (Saccostrea commercialis) were transplanted in various sites along the North Queensland coast and analyzed for two metals of potentially anthropogenic origin (Cd, Zn). To provide additional information, naturally occurring Black Lip oysters (Saccostrea echinata) were also collected at the transplantation sites. The study demonstrated that the oysters species transplanted are good bioindicators of these metal concentrations in tropical waters, sensitive to variations in the environment at concentrations which are much smaller than pollution signals commonly reported for temperate waters. Three transplant experiments were carried out from May 1999 to February 2000. Milky oysters transplanted to offshore areas (Orpheus Is., Kelso Reef) accumulated Cd in the soft parts whereas oysters sampled from cages placed in Ross Creek and the Herbert River estuaries showed a decrease in Cd concentration, which resulted from an increase in dry weight. Dry weight appeared to be an important covariant affecting Cd concentration in the oysters whereas it does not unambiguously affect Zn concentrations. For the duration of the experiments, oysters sampled from the Magnetic Is. reference site showed effectively constant Cd concentrations and total Cd contents which indicates that any seasonal cycle affecting metal concentration is weak. It was found that Cd accumulation in oysters increased as ambient dissolved Cd concentration decreased, from which it was concluded that for these oysters, the predominant source of Cd was from the particulate phase rather than the dissolved phase.