Isao Koike

Importance of foraminifera for the formation and maintenance of a coral sand cay: Green Island, Australia

Abstract CaCO3 production by reef-building organisms on Green Island Reef in the Great Barrier Reef of Australia is estimated and compared with the contribution of benthic foraminifera to the sediment mass of the vegetated sand cay. Major constituents of the cay are benthic foraminifera (mainly Amphistegina lessonii, Baculogypsina sphaerulata, and Calcarina hispida), calcareous algae (Halimeda and coralline algae), hermatypic corals, and molluscs. Among these reef-building organisms, benthic foraminifera are the single most important contributor to the sediment mass of the island (ca. 30% of total sediments), although their production of CaCO3 is smaller than other reef-building organisms. Water current measurements and sediment traps indicate that the velocity of the current around Green Island is suitable for transportation and deposition of foraminiferal tests. Abundant foraminifera presently live in association with algal turf on the shallow exposed reef flat, whose tests were accumulated by waves resulting in the formation and maintenance of the coral sand cay.

Significance of groundwater nitrogen discharge into coral reefs at Ishigaki island, southwest of Japan

Abstract. Groundwater discharge from adjacent terrestrial areas can be a potentially important nutrient source to coastal coral reefs, since adjacent lands are often overlaid with permeable bedrock such as limestone. The quantity of groundwater nitrogen discharged into the Shiraho and Kabira coral reefs from their namesake watersheds on Ishigaki Island southwest of the Ryukyu Islands, Japan (24°19′–37′N, 124°4′–21′E) was monitored. These watersheds were subject to different types of nitrogen loading. The groundwater nitrogen discharge was compared by two independent methods, one based on measuring dissolved inorganic nitrogen (DIN) concentrations in the groundwater near the coastline, the other by estimating nitrogen loading from various land uses within the watershed. For a common watershed, the two methods agreed within a factor of two. The Shiraho reef received 4- or 5.5-fold more nitrogen than the Kabira reef. Groundwater discharge contributes significantly to the reef nitrogen budget, and is potentially a key factor controlling the biomass and succession of aquatic vegetation of the reefs.

Hydrographic features of the deep water of the Bering Sea—the sea of Silica

Abstract The deep water of the Bering Sea contains concentrations of dissolved silicate up to 240 μg at. Sil−1. Nitrate concentrations are less than in the North Pacific at the depths with the same oxygen contents. The rates of chemical and biochemical reactions occurring in the deep water (below 2km) were estimated from hydrographic data by applying a modified one-dimensional model. Oxidation of organic matter in the oxygenated water column of the Bering Sea was twice that of the North Pacific. Silicate regeneration, or dissolution of biogenic opal and denitrification, or bacterial nitrate reduction to gaseous nitrogen, on and in the bottom sediments of the deep Bering Sea basin were calculated to be 212 and 20 mg at.m−2 yr−1, respectively. These values are consistent with the ones estimated from vertical profiles of dissolved silicate and nitrate in the interstitial water of the sediments. The chemical anomaly observed in deep water of the Bering Sea can be produced by these reactions in the bottom sediments. The decomposition of organic matter in anoxic sediments accounts for about 8% of the total organic matter decomposing in the water column below 2km and in the sediments.

Estimates of denitrification in sediments of the Bering Sea shelf

Denitrification, i.e. anaerobic reduction of nitrate or nitrite to gaseous nitrogen, in the surface sediments of the Bering Sea was estimated using a 15N-tracer method. N2 production is apparently controlled by the supply of nitrate and nitrite to the sediments. The average rate of N2 production in three locations was 1.2 ng-atoms N (g dry weight of sediment)−1 h−1. The rate of nitrate reduction estimated from the vertical distribution of nitrate in the sediments using a one-dimensional diffusion model agreed well with observed rates of N2 production. The annual loss of combined nitrogen by denitrification in the Bering Sea shelf was estimated to be 5 × 1011 g.

High potential activity of extracellular alkaline phosphatase in deep waters of the central Pacific

Abstract We found high potential activities of alkaline phosphatase associated with particles (0.2 μm or greater size fraction) in deep waters (1000–4000 m) of the central Pacific Ocean. The potential enzyme activity at depth (0.03 – 0.3 nM h −1 ) was up to 50% of that at the surface (0–125 m). In contrast, activities of α- and β- d -glucosidase in the deep layer were low (generally less than 1 % of those in the upper layer), yielding up to two orders of magnitude difference in the ratio of alkaline phosphatase and α- and β- d -glucosidase activities with depth. It is unlikely that the phosphatase is actively produced by microorganisms inhabiting the deep-sea environment, where labile organic carbon supply is limited and phosphate concentration is high (2.4 – 3.0 AM). Instead, deep-water phosphatase is probably supplied by rapidly sinking particles and their subsequent fragmentation and dissolution. Different distributions of phosphatase and glucosidase indicate that sinking particles of phytoplankton origin are an important source of alkaline phosphatase enzymes in the deep sea.

Nitrogen metabolism by heterotrophic bacterial assemblages in Antarctic coastal waters

Field studies to examine the in situ assimilation and production of ammonium (NH4+) by bacterial assemblages were conducted in the northern Gerlache Strait region of the Antarctic Peninsula. Short term incubations of surface waters containing 15N-NH4+ as a tracer showed the bacterial population taking up 0.041–0.128 μg-atoms Nl−1d−1, which was 8–25% of total NH4+ uptake rates. The large bacterial uptake of NH4+ occurred even at low bacterial abundance during a rich phytoplankton bloom. Estimates of bacterial production using 3H-leucine and -adenine were l.0μgCl−1 d−1 before the bloom and 16.2 μg Cl−1 d−1 at the bloom peak. After converting bacterial carbon production to an estimate of nitrogen demand, NH4+ was found to supply 35–60% of bacterial nitrogen requirements. Bacterial nitrogen demand was also supported by dissolved organic nitrogen, generally in the form of amino acids. It was estimated, however, that 20–50% of the total amino acids taken up were mineralized to NH4+. Bacterial production of NH4+ was occurring simultaneously to its uptake and contributed 27–55% of total regenerated NH4+ in surface waters. Using a variety of 15N-labelled amino acids it was found that the bacteria metabolized each amino acid differently. With their large mineralization of amino acids and their relatively low sinking rates, bacteria appear to be responsible for a large portion of organic matter recycling in the upper surface waters of the coastal Antarctic ecosystem.

Distribution of dissolved organic carbon in the East China Sea

Abstract A high-temperature combustion (HTC) method with high precision (±0.3– 1.2 μM ) was applied to measure the distribution of dissolved organic carbon (DOC) in the East China Sea. The seawater samples were collected near the shelf edge in the autumn (October–November 1995) and along a transect on the shelf (the PN line) in the spring (April 1996). In the surface mixed layer, DOC concentrations varied from approximately 65– 75 μM , within the range generally reported for other oceanic regions. In general, the distribution of DOC seemed to be controlled by hydrography (σt) rather than biology (chlorophyll a). However, our method also detected small variations (usually μM ) within the surface mixed layer, implying photochemical degradation of DOC in the top surface and a temporal accumulation of DOC of intermediate reactivity. Depth-integrated DOC increased by 13 g C m −2 from the autumn to the spring in the 100– 200 m layer around the shelf edge, comparable to the annual particle flux from the euphotic zone, suggesting substantial downward export of DOC in this area. Terrestrial input of DOC was estimated to be 4.8×10 12 g C yr −1 in the shelf area relatively close to the river mouth of the Changjiang.

Upwelling plumes in sagami bay and adjacent water around the Izu Islands, Japan

Water plumes, 20 km long or less, identified by low temperature, high salinity and high nutrient concentrations, were observed on the eastern side of Izu Islands where the Kuroshio Current or its branch flowed eastward. The T-S diagrams and the vertical profiles of oceanographic variables indicated that the water plumes resulted from the upwelling of subsurface water. A newly formed plume, characterized by a sharp temperature front and high nutrient concentrations, contained less chlorophyll than did old plumes. It is suggested that the upwelling plumes are maintained for a period long enough to allow luxuriant growth of phytoplankton.

Temporal focusing of nitrogen release by a periodically feeding herbivorous reef fish

The reef-dwelling jewel damselfish Plectroglyphldodon lacrymatus (Quoy & Gaimard) exhibited a marked diel periodicity in nitrogenous excretion and defecation rates. This temporal “focusing” of soluble and particulate-faecal nitrogen release on the latter part of the day correlated strongly with the pattern of feeding activity. There was a lag after feeding began each day of ≈ 5–6 h before defecation accelerated, but no such lag was apparent in the case of excretion. Both excretion and defecation, however, did lag behind feeding at the end of the day, for they remained above their respective minimum rates for ≥ 8 h after feeding had ceased. Gut evacuation rates were variable, while the excretion of nitrogenous waste appeared to take ≈24 h and to be subject to a physiological rhythm. Defecation, most of which occurred at a single site in each territory, was far more important in the generation of nitrogen by the fish than was excretion. In laboratory experiments the faeces took up ammonium, but in the field most faeces were probably removed quickly by ophiocomine brittlestars and pulled down into interstices of the reef. Nearly all of the nitrogen egested, but perhaps only one-third of that excreted, was transferred initially to the reef framework below the fishs territory.

The role of the sea urchin,Tripneustes gratilla(Linnaeus),in decomposition and nutrient cycling in a tropical seagrass bed

The sea urchin,Tripneustes gratilla, which feeds mainly on living leaves of the seagrass,Thalassia hemprichii, was studied in its habitat on the southern coast of Papua New Guinea, and its roles in decomposition and nutrient cycling in a seagrass bed were assessed through the excretion of ammonium and metabolism of feces produced by the sea urchin. Carbon content of the fresh feces (21% of dry weight) was similar to that of intact dead leaves of the same species (22–23%). Carbon/nitrogen and carbon/phosphorus ratios of the feces (21.7 and 466, respectively), however, were significantly lower than those of the dead leaves (25.9–27.7 and 656–804, respectively), indicating that the feces retain more nitrogen and phosphorus in comparison with carbon.Net consumption of ammonium and orthophosphate typically concurred with oxygen consumption during dark incubation of both the dead leaves and the sea urchin feces. Compared with the same oxygen consumption rate, however, the dead leaves consumed more orthophosphate than the feces. Under sunlight, dead leaves showed a net accumulation of carbon by epiphytic algae, while the feces showed a carbon loss.Ammonium excretion by this sea urchin (1.7–5.4 mg nitrogen/individual/day) would thus appear to make a significant contribution to nitrogen recycling since biological communities associated with dead leaves and sea urchin feces tend to demand an external supply of nitrogen, such as ammonium.