Relative sea-level change regulates organic carbon accumulation in coastal habitats.

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.

Historical δ15N records of Saccharina specimens from oligotrophic waters of Japan Sea (Hokkaido).

Historically Saccharina spp. beds occurred along the west coast of Hokkaido, an oligotrophic area, and were commercially exploited. Currently extensive commercial Saccharina spp. beds do not form due to nutrient limitations. Here, we postulate that nutrients assimilated by paleo-Saccharina spp. beds may have been derived from spawning herrings (Clupea pallasii) acting as organisms that formed a vector from their feeding grounds (Okhotsk Sea and Pacific Ocean) to their spawning area (west coast of Hokkaido, Japan Sea). To test this hypothesis we examined stable nitrogen isotope ratios (δ15N) of 100- to 135-year-old Saccharina specimens preserved at the Herbarium (Hokkaido University Museum). δ15N values of the paleo-Saccharina specimens collected from this region were in the range of 10‰, which is significantly higher than the current 3-7‰ in freshly sampled Saccharina spp. This high δ15N indicates that spawning herring (Clupea pallasii) had potentially been a significant source of dissolved inorganic nitrogen (DIN) absorbed by Saccharina, acting as an organism forming a vector for transporting nutrients from eutrophic to oligotrophic coastal ecosystems. Our findings support the hypothesis of so-called "herring-derived nutrients."

Net uptake of atmospheric CO2 by coastal submerged aquatic vegetation.

'Blue Carbon', which is carbon captured by marine living organisms, has recently been highlighted as a new option for climate change mitigation initiatives. In particular, coastal ecosystems have been recognized as significant carbon stocks because of their high burial rates and long-term sequestration of carbon. However, the direct contribution of Blue Carbon to the uptake of atmospheric CO2 through air-sea gas exchange remains unclear. We performed in situ measurements of carbon flows, including air-sea CO2 fluxes, dissolved inorganic carbon changes, net ecosystem production, and carbon burial rates in the boreal (Furen), temperate (Kurihama), and subtropical (Fukido) seagrass meadows of Japan from 2010 to 2013. In particular, the air-sea CO2 flux was measured using three methods: the bulk formula method, the floating chamber method, and the eddy covariance method. Our empirical results show that submerged autotrophic vegetation in shallow coastal waters can be functionally a sink for atmospheric CO2. This finding is contrary to the conventional perception that most near-shore ecosystems are sources of atmospheric CO2. The key factor determining whether or not coastal ecosystems directly decrease the concentration of atmospheric CO2 may be net ecosystem production. This study thus identifies a new ecosystem function of coastal vegetated systems; they are direct sinks of atmospheric CO2.

Food sources for Ruditapes philippinarum in a coastal lagoon determined by mass balance and stable isotope approaches.

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.

Is there any seasonal variation in marine nematodes within the sediments of the intertidal zone?

The sediment parameters and nematode assemblages in the intertidal zone of the Hichirippu shallow lagoon, Hokkaido, Japan, were investigated. The objectives of this study were to observe the seasonal variation in the nematodes in the sediment, and to investigate the relationships between the nematodes and environmental factors. Samples were collected bi-monthly from five stations on the tidal flat from April 2003 to February 2004. It was found that the sediment parameters (Chl a concentration, AVS, TOC and TN contents) varied throughout the 10-month study. Fifty-four species of nematodes were found in the study area. The density and biomass of the nematodes varied in accordance with the sediment temperature during the sampling period. In this study, there was a seasonal variation in the nematode assemblage found in the intertidal zone of this shallow lagoon. The important factors affecting this variation were sediment temperature, and food competition among the nematodes themselves. The seasonal variation of the nematode also showed a relationship with the Chl a concentration in the sediment during the sampling period.

The influence of environmental variability on silicate exchange rates between sediment and water in a shallow-water coastal ecosystem, the Seto Inland Sea, Japan.

Silicate regeneration was determined in a shallow-water coastal ecosystem (Shido Bay, the Seto Inland Sea, Japan) during 1999-2000. The present study was carried out directly by core incubation and by determining gradients of dissolved silicate (DSi) in the pore water. Incubated fluxes ranged from 25.5 to 132.6 mgSim(-2)d(-1), and were 1.6-21.6 times greater than diffusive fluxes (5.4-43.3 mgSim(-2)d(-1)). The disparity between fluxes measured by core incubation and modeling pore water indicated that other physical, chemical or biological processes, in addition to diffusion of DSi from below, contribute to DSi fluxes measured during the incubation of undisturbed cores. A regression analysis revealed that water temperature and salinity explained 24% and 23%, respectively, of season to season variability in incubated fluxes. Microphytobenthos was responsible for 37% of the variability in measured rate of DSi fluxes, with greatly reducing DSi release rates due to their own DSi demand. Moreover, the inverse relationship between the DSi fluxes and biogenic silica (Bio-Si) concentrations in the surface sediment, suggested that about 41% of the variability in the DSi fluxes were explained by Bio-Si concentrations in the surface sediment. As a result, Shido Bay showed silicate regeneration of incubated cores to be a consequence of Bio-Si dissolution depending on microphytobenthos, temperature and salinity, while diffusive fluxes appeared to be limited by DSi in the pore water. An annual average of DSi flux (68.7+/-32.9 mgSim(-2)d(-1)) from the sediments to the water column corresponds to 38% of DSi, required for primary production by phytoplankton in Shido Bay.

Estimation of particulate organic carbon flux in relation to photosynthetic production in a shallow coastal area in the Seto Inland Sea.

Sediment trap experiments were carried out three times from 1999 to 2000, in the western part of the Seto Inland Sea (Suo-Sound), Japan. We investigated both the particulate flux and the composition of chemical substances in the sediment trap samples. Based on the results, we discuss the origin of particulate organic carbon (POC) collected by the sediment traps in a coastal area. Moreover, we purposed to estimate the flux of the portion of the POC that is derived from phytoplankton photosynthesis. The fluxes of POC varied between 677 and 3424 mgC m(-2) d(-1). Significant positive correlations between POC and aluminum (Al) fluxes suggested that these components show almost the same behaviour. The mean value of the Al flux was about eight times higher than that of Al burial rates on the sediment surface. Therefore, it seems that the POC flux observed with the sediment traps was considerably overestimated. Moreover, judging from the fact that Al is a typical terriginous element, it seems that most of the POC collected in the sediment traps derived from the re-suspended surface sediment or sediment transported laterally from shallow flanks such as intertidal mudflats. The fluxes of chlorophyll a (Chl a) were independent of the POC fluxes, and a relatively consistent correlation was found between Chl a abundance in the water column and the Chl a flux. Moreover, surface sediment Chl a content was approximately 100 times lower than that of suspended matter. Therefore, resuspension and terriginous contributions to Chl a collected in sediment traps are likely to be negligible. The POC content in the trap samples varied between 22.4 and 70.7 mg g(-1) dry weight. The variations of POC contents were positively correlated with the Chl a contents: POC(mg g(-1))=76.5 x Chl a(mg g(-1)) + 26.0 (r=0.95, p<0.01, n=9). This result shows that POC contents strongly corresponded with phytoplankton and their debris. It was also considered that the fraction of POC derived from phytoplankton primary production could be estimated as Chl a content times a certain factor. In this study, we estimated the flux of the portion of the POC originating from phytoplankton production by multiplying the Chl a fluxes by 76.5 (the mean POC:Chl a ratio in the trap samples). These values varied between 308 and 758 mgC m(-2) d(-1), and accounted for 35.1+/-21.2% of total POC flux. Although the amount of POC that originates from phytoplankton photosynthesis was a small portion of total POC flux, it seems to be a large portion of potential primary production in the water column.