Historical delta(15) N 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 (delta N-15) of 100-to 135-year-old Saccharina specimens preserved at the Herbarium (Hokkaido University Museum). delta(15) N 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 delta(15) N 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

Estimation of microphytobenthic resuspension fluxes in a shallow lagoon in Hokkaido, Japan

We conducted field sampling in a subarctic shallow lagoon (Hichirippu Lagoon) in the eastern part of Hokkaido, Japan. We investigated the chemical composition of the water column, sediment, and sinking particles collected by the sediment trap. The standing stock of chlorophyll a (Chl-a) in the water column and surface sediment were 0.4 to 9.3 and 35.9 to 184 mg m−2, respectively. Using stable isotope analysis, the contribution rate of microphytobenthos to the sinking particles was found to range from 63 to 74%. The average Chl-a content in the sinking particles was significantly lower than that of the water column. Our results suggest that the contribution of phytoplankton present in the water column to the Chl-a collected in the traps is likely to be negligible. We assumed that the Chl-a fluxes obtained in this study were microphytobenthic resuspension fluxes. The daily flux of Chl-a accounted for 47.0 to 1,270% of the total standing stock of Chl-a in the water column. The mean relative percentage of daily Chl-a flux divided by the standing stock of Chl-a in the sediment was 6.5%, which indicates that approximately 7% of the microphytobenthos present in the sediment was resuspended, and 93% of the total succeeded at escaping the winnowing action. Although the resuspension phenomenon had little effect on the population of microphytobenthos, the resuspended microphytobenthos had a major impact on the total micro algal biomass in the water column. This is the first direct estimate of microphytobenthic resuspension flux in shallow estuaries

Effects of Temperature on the Germination of Marine Phytoflagellate Cysts

The effect of temperature on the germination of some phytoflagellate cysts was studied in two different temperature conditions, step-gradient temperature and constant temperature. The highest number of newly germinated cell of Scrippsiella spp., Alexandrium spp., Protoperidinium spp., Gyrodinium spp., Gymnodinium spp., and Diplopelta spp. was observed after incubation 2 or 3 days when the initial temperatures were 13 and/or 19 ℃, but it took at least 5 days at the initial incubation temperature of 10 ℃. The germination rates of phytoflagellate cyst under optimum window should be faster than the other temperature ranges. The optimum temperature for cyst germination of Alexandrium spp., Protoperidinium pellucidum Bergh and Protoperidinium spp. was considered to be at 16 ℃. For Scrippsiella spp., Gymnodinium spp. (small) and Gymnodinium spp., the optimum temperature for their cyst germination was considered to be 13 ℃, whereas that for Chattonella spp. could be as high as 25 ℃

Seasonal and interannual patterns of intertidal microphytobenthos in combination with laboratory and areal production estimates

From April 1994 to December 1997, we studied the microphytobenthic assemblages in surface (0 to 0.5 cm) and subsurface (0.5 to 2 cm) sediments at spring low tide along a transect of 5 stations in an estuarine sandflat of the Seto Inland Sea, Japan. At the innermost sampling station, microphytobenthos biomass (chl a) was also investigated in a vertical profile to 10 cm depth from December 1994 to April 1996. The chl a contents at the 2 uppermost layers were well correlated with each other, with a mean decrease of 34% from the surface to subsurface layer. Chl a tended to decrease rapidly through the vertical profile and was reduced to 3.2 ± 1.4% SD in the 9 to 10 cm layer. There was a progressive decrease in the chl a content every year in fall and the occurrence of major peaks in early spring and/or summer. This was accompanied by a significant increase in microphytobenthos biomass from 1994 to 1995 and from 1995 to both 1996 and 1997. The microphytobenthos biomass in surface sediments (mean of 5 stations) ranged between 27.7 (October 1994) and 120 mg chl a m–2 (July 1997), or between 3.9 (November 1994) and 20.3 μg chl a g–1 dry wt (July 1996). Annual mean (1995 to 1997) biomass was 72.3 ± 27.1 mg chl am–2 and 11.0 ± 4.3 μg chl a g–1 dry wt. These values rank in the mid-upper range of microphytobenthic biomass for intertidal sediments. In addition to the field investigations, we conducted laboratory experiments on a dominant diatom species, Navicula sp. The photosynthetic rate of Navicula sp. was saturated at a light intensity of 165 μE m–2 s–1 at 21°C. No photoinibition was found at higher light intensities up to 400 μE m–2 s–1. The relationship between temperature and photosynthetic rate was positive and linear within a temperature range between 10 and 35°C at 55 μE m–2 s–1. Areal ‘potential’ primary production of microphytobenthos was between 0.32 (December 1994) and 3.0 gC m–2 d–1 (July 1997), with an annual mean of 1.2 gC m–2 d–1. Uni- (summer) or bi-modal (spring and summer) peaks of microphytobenthos biomass and primary production highlighted a marked interannual variability. Marked seasonal patterns were also recognizable, with primary production of microphytobenthos significantly higher both in spring and summer than in winter and fall

Role of tidal flat in material cycling in the coastal sea

A simple tidal flat model with pelagic and benthic ecosystems was developed in order to analyze the nitrogen cycling in an inter-tidal flat of the Seto Inland Sea, Japan. After the verification of calculation results with the observed results in water quality and benthic biomasses, the role of this tidal flat in nitrogen cycling was evaluated from the viewpoint of water quality purification capability. When there is no suspension feeder in the tidal flat, the water quality purification capability of this tidal flat becomes lower because the outflow of organic nitrogen increases compared to the present case, and the red tides may be generated

High tolerance of phytoplankton for extremely high ammonium concentrations in the eutrophic coastal water of Dokai Bay (Japan)

The tolerance of phytoplankton in Dokai Bay for an extremely high ammonium concentration in culture media has been studied. Six species of phytoplankton, three diatoms (two clones of Skeletonema sp. and Chaetoceros sp.) and three flagellates (Heterosigma akashiwo, Chattonella antiqua and Karenia mikimotoi) were grown in various concentrations of NH4Cl. The results suggested that high ammonium concentrations had negative effects on phytoplankton growth. Non-indigenous species in Dokai Bay, Japan, C. antiqua and K. mikimotoi, were unable to grow at 200 and 150 μ M, respectively. Growth rates of Skeletonema sp. isolated from Harima Nada (Seto Inland Sea, Japan), Chaetoceros sp. and H. akashiwo were reduced significantly at higher ammonium concentrations compared to the control treatment. However, such a high ammonium concentration of even 1,500 μ M could not produce a significant adverse effect on the growth rate of Skeletonema sp. isolated from Dokai Bay. Furthermore, the maximum chlorophyll fluorescence of tested species was also gradually decreased with an increase in ammonium concentration. The influence of a high ammonium level on phytoplankton growth observed in this study confirmed the phytoplankton species composition observed in Dokai Bay. Our results suggested that such a high ammonium concentration was an important factor in determining the species composition of the phytoplankton assemblage in that bay