Because coastal habitats store large amounts of organic carbon (Corg), the conservation and restoration of these habitats are considered to be important measures for mitigating global climate change. Although future sea‐level rise is predicted to change the characteristics of these habitats, its impact on their rate of Corg sequestration is highly uncertain. Here we used historical depositional records to show that relative sea‐level (RSL) changes regulated Corg accumulation rates in boreal contiguous seagrass–saltmarsh habitats. Age–depth modeling and geological and biogeochemical approaches indicated that Corg accumulation rates varied as a function of changes in depositional environments and habitat relocations. In particular, Corg accumulation rates were enhanced in subtidal seagrass meadows during times of RSL rise, which were caused by postseismic land subsidence and climate change. Our findings identify historical analogs for the future impact of RSL rise driven by global climate change on rates of Corg sequestration in coastal habitats.
The relationship between the food demand of a clam population (Ruditapes philippinarum (Adams & Reeve 1850)) and the isotopic contributions of potential food sources (phytoplankton, benthic diatoms, and organic matter derived from the sediment surface, seagrass, and seaweeds) to the clam diet were investigated. In particular, we investigated the manner in which dense patches of clams with high secondary productivity are sustained in a coastal lagoon ecosystem (Hichirippu Lagoon) in Hokkaido, Japan. Clam feeding behavior should affect material circulation in this lagoon owing to their high secondary productivity (ca. 130 g C m−2 yr−1). Phytoplankton were initially found to constitute 14–77% of the clam diet, although phytoplankton nitrogen content (1.79–4.48 kmol N) and the food demand of the clam (16.2 kmol N d–1) suggest that phytoplankton can constitute only up to 28% of clam dietary demands. However, use of isotopic signatures alone may be misleading. For example, the contribution of microphytobenthos (MPB) were estimated to be 0–68% on the basis of isotopic signatures but was subsequently shown to be 35±13% (mean ± S.D.) and 64±4% (mean ± S.D.) on the basis of phytoplankton biomass and clam food demand respectively, suggesting that MPB are the primary food source for clams. Thus, in the present study, the abundant MPB in the subtidal area appear to be a key food source for clams, suggesting that these MPB may sustain the high secondary production of the clam.
Abstract:The small gastropod, Lacuna decorata Adams, living on macrophytobenthos or surface sediment, is one of the most dominant species of macrozoobenthos in Hichirippu lagoon covered with seagrass and macroalgae, eastern Hokkaido, Japan. We measured the standing stocks of primary producers and macrozoobenthos, and determined the stable carbon and nitrogen isotope ratios of the primary producers and L. decorata. With these results, we identify the main food items for L. decorata and discuss the feeding strategy of the small gastropod. This gastropod occupied about 64% in density and about 25% in biomass of the macrozoobenthos at all six sampling stations in the lagoon. It occurred densely on the surface of the sediment with dense patches of benthic microalgae (BMA), which contained extremely high levels of Chl.-a between 84 to 226 mg m Ϫ2 throughout the period of this study. Nevertheless, the stable isotope signatures of carbon and nitrogen of this gastropod clearly show the direct utilization of organic matter derived from seagrass, Zostera japonica, in the areas where the seagrass luxuriated. However, it shows also a flexible feeding strategy in food preference. It fed green algae such as Ulva pertusa and Urospora wormskioldii in the areas where the seagrass grew scarcely.Key words: food resource, gastropod, green algae, Lacuna decorata, lagoon, seagrass, stable isotope, Zostera japonica * Corresponding author: Rumiko Kajihara; E-mail, rmk-kjhr@ees.hokudai.ac.jpThe description of feeding activities of the dominant species in the macrobenthic communities is important to follow the material or energy transportation from the primary producers to the primary consumers (the secondary producers) in the benthic ecosystem. However, it is often not easy to observe the feeding activities of small macrobenthic animals such as L. decorata and to identify their main food items by direct observation of feeding behaviors. For the description of food chain in the marine ecosystem, the techniques of stable isotope analysis have become popular recently, since the accumulation of empirical results on the carbon and nitrogen stable isotope ratios (d 13 C and d 15 N) has revealed that each primary producer, such as phytoplankton, benthic microalgae, seagrass and macroalgae, has a unique range of the stable carbon isotope ratios (Fry & Sherr 1984, France 1995, animals have a slightly enriched carbon stable isotope ratio (0.8Ϯ1.1‰, d13 C) to those of their main food items, and the enrichment of nitrogen stable isotope ratio between two different trophic levels in the food chain is relatively constant (d 15 N: 3.4Ϯ1.1‰) (DeNiro & Epstein 1978, Minagawa & Wada 1984. The determination of carbon and nitrogen isotope ratios of the small gastropod, L. decorata, also seems to be helpful for identification of main food items, particularly because it occurs in the two different habitats on the sediment of the sea floor and on the leaves of seagrass or the thalli of macroalgae.We have studied the structure of lagoon ecosystems in eastern Ho...
Previous estimations of nutrient mineralization in the water column by infaunal bivalves might have been overestimated because of underestimation of the uptake process by microphytobenthos in the field. We conducted field surveys of environmental conditions and quantitative sampling of Ruditapes philippinarum in a shallow lagoon system (Hichirippu Lagoon, eastern Hokkaido, Japan) in August 2006. We recorded the spatial distribution pattern and the molar ratio of dissolved inorganic nutrients to determine the limiting nutrients for microphytobenthos, to evaluate the input and output of nutrients at the entrance of the lagoon station, and to estimate potential nutrient mineralization by R. philippinarum. Our aim was to reevaluate the nutrient mineralization process by infaunal bivalve species. In this study, the mean standing stock of microphytobenthos inhabiting surface sediment (5 mm thick) on the tidal flats was 100 times higher than that of phytoplankton (1 m depth). Low N/P and high Si/N ratios (mean = 2.6 and 17.6, respectively) near the entrance of the lagoon compared to those of microphytobenthos (N:P:Si = 10.1:1:18) clearly suggest N deficiency. The flux of NH 4 -N coming into the lagoon was 3.4 kmolN d −1 , and the flux out was −3.7 kmolN d −1 . Thus, assuming that there would have been no phytoplankton and microphytobenthos uptake during the day, 0.3 kmolN d −1 of NH 4 -N was produced within the lagoon. However, the NH 4 -N mineralization rate of the clams has been estimated to be approximately 7.7 ± 6.8 kmolN d −1 . Thus, 96% (
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