Topic outline

  • General

  • Background

    • Primary productivity in coastal ecosystem supports various components of marine biodiversity and ecosystem functions. This affects not only higher trophic levels in the pelagic layers, but also benthic consumers including important fisheries targets (e.g. bivalves, decapod crustaceans). Coastal ecosystems are affected by oceanic dynamics, terrestrial input and geographical (topographical) structures, which make spatial variation in communities and production greater and more complex than offshore pelagic ecosystems. Eastern Hokkaido is predominantly influenced by Coastal Oyashio Water (COW) which brings low temperature and low salinity. Spring phytoplankton blooms occur every year in the COW, supporting high productivity and ultimately various seafood resources. Although the extent of global coastal wetlands is seeing overall declines due to various anthropogenic activities, pristine freshwater/brackish wetlands remain in eastern Hokkaido. These buffer against the direct river discharge of terrestrial matter through rivers. Continued protection and conservation of wetlands may benefit coastal productivity and sustainability. Therefore, surveys of areas that have coastal wetlands are important. 

      The main objective of this article is to examine spatial variation of diatoms along nearshore areas of Biwase and Hamanaka Bays that are implied to display terrestrial effects from Kiritappu Wetland, Eastern Hokkaido. We conducted in situ surveys measuring seawater temperature, salinity, nutrients concentration, and colored dissolved organic matter (CDOM) absorption as indicators of wetland-influenced river discharge at the surface to investigate the spatial variation in abiotic coastal water properties in nearshore areas. In addition to observations on diatom taxa composition, we measured both total and size-fractionated chlorophyll (Chl) a concentrations to act as biotic environmental factors of the study area.

  • Study area

    • Hamanaka town is a key fisheries town in eastern Hokkaido. The coastal area of Hamanaka town is divided into Biwase Bay and Hamanaka Bay, both of which face Kiritappu Wetland. Kiritappu Wetland is the third largest wetland of Japan (3,168 ha) and internationally protected by the Ramsar Convention. Biwase Bay is a semi-enclosed bay directly impacted by river-discharge of Biwase River running through Kiritappu Wetland. Hamanaka Bay is open to the Pacific and is therefore more affected by COW than the wetland. Spatial comparisons of these two bays will give insight into how much the effects of wetland extends to coastal areas. 


  • Material and Methods

    • Seawater sampling was conducted at a total of 11 stations in Biwase and Hamanaka Bays on June 23, 2014 in the timing of flood tide. Sampling stations were divided into four areas as follows: Area 1, Biwase Bay near the Biwase River mouth (St.1-2); Area 2, Biwase Bay facing Kiritappu Wetland (St.3-5); Area 3, Hamanaka Bay facing Kiritappu Wetland (St. 6-8); Area 4, Hamanaka Bay, apart from Kiritappu Wetland and directly affected by Coastal Oyashio Water (St.9-11). Biwase Bay and Hamanaka Bay are geographically divided by peninsula of Cape Kiritappu. Stations were along the 5-m isobath except for St.1 (1.8 m). Before seawater sampling, seawater temperature and salinity were measured with a conductivity, temperature, and depth (CTD) sensor (RINKO profiler ASTD102; JFE Advantech, Nishinomiya, Japan). Next, samples for the measurement of nutrient concentration, total and size-fractionated Chl a concentrations, cell density and composition of diatoms, absorption coefficient of colored dissolved organic matter (CDOM) were collected from the surface using an acid-cleaned bucket.


  • Results and Discussion


    • The water temperature in Area 4 was relatively lower than those in other areas. Even though there was a significant spatial variation by ANOVA, the post-hoc test showed no significant difference between each pair of areas. Salinity was significantly lower in Area 1 than Area 4, while no significant difference was found in other pairs of areas. Chlorophyll a concentration ranged from 0.68 μg/L to 2.33 μg/L. The lowest Chl a concentration was found in Area 1. The aCDOM(443) was significantly higher in Area 1 than in the other three areas.


    • The total amount of nitrite and nitrate concentrations was significantly higher in Area 4 than in Areas 2 and 3. In contrast, neither ammonium nor phosphate concentrations showed significant differences among areas. Silica concentration in Area 1 was significantly higher than in the other areas.


    • Micro-sized (>10 μm) phytoplankton dominated the total phytoplankton community in Area 2, while the composition of nano- (2–10 μm) and pico-sized (0.7–2 μm) phytoplankton accounted for 25.5% and 25.3%, respectively. In Area 3, the composition of micro- (39.1%), nano- (24.6%), and pico- (36.3%) sized phytoplankton were similar. On the other hand, the predominance of micro-sized phytoplankton (76.2%) was found in Area 4.


    • A total of 34 genera were observed from four different areas. Diatom composition was more diverse in Biwase Bay than in Hamanaka Bay. There were 21 and 28 diatom genera counted in Areas 1 and 2, respectively, whereas 11 and 17 genera were found in Areas 3 and 4. The abundance in Area 1 was significantly higher than those in the other three areas, and diatoms in Area 2 were significantly more abundant than Areas 3 and 4. There was no significant difference between Areas 3 and 4.


    • Cluster analysis based on the diatom genera and their cell densities classified the areas into Biwase Bay (Areas 1 and 2) and Hamanaka Bay (Areas 3 and 4). In addition, pennate diatoms were more abundant in Biwase Bay, while centric diatoms were abundant in Hamanaka Bay. The most dominant diatom genus was Cocconeis in Biwase Bay (88.7% at Area 1, 56.9% at Area 2), and Thalassiosira in Hamanaka Bay (42.8% at Area 3, 57.9% at Area 4).

    • Diatom composition along the nearshore areas of Biwase Bay and Hamanaka Bay, eastern Hokkaido, Japan, showed significant spatial variation in abundance and taxa within a spatial extent of 10–15 km. Environmental variables such as temperature, salinity and nutrient contents, and geographical feature of the coast were related to our observed variation. The variation in the environmental factors appear to be strongly affected by both the Coastal Oyashio Water and terrestrial input by Biwase River and surrounding Kiritappu Wetland. Geographical influences are related to the historical connectivity of pelagic waters that are now divided by the peninsula laying between Biwase and Hamanaka Bays. Although this study is based on just a snapshot of data, it is the first dataset collected for Hamanaka coastal water. The obtained data can be used as baseline data for future research, including the evaluation of impacts of ongoing climate change on coastal ecosystems, which is expected to heavily affect the coastal ecosystems and biodiversity of this region.

  • Funding information

    • This work was supported by the Environment Research and Technology Development Funds (S-9: Integrative observations and assessments of Asian biodiversity; S-15: Predicting and Assessing Natural Capital and Ecosystem Services (PANCES)) of the Environmental Restoration and Conservation Agency of Japan, grant-in-aid by Hokkaido e-water Project, and Hirose Foundation.