Dynamics of Nutrients and Colored Dissolved Organic Matter Absorption in a Wetland-Influenced Subarctic Coastal Region of Northeastern Japan: Contributions From Mariculture and Eelgrass Meadows

Abstract

Coastal oceans interacting with terrestrial ecosystems play an important role in biogeochemical cycles. It is therefore essential to research land–ocean interactions for further understanding of the processes influencing nutrients dynamics in coastal areas. We investigated the seasonal and spatial distribution of nutrient concentrations and light absorption coefficients of colored dissolved organic matter (CDOM), non-algal particles (NAP), and phytoplankton in a wetland-influenced river–eelgrass meadows–coastal waters continuum in the protected and semi-enclosed coastal sea of Akkeshi-ko estuary (AKE) and Akkeshi Bay (AB), Japan from April 2014 to February 2015. The mixing dilution lines of the CDOM absorption coefficient at 355 nm [aCDOM(355)] relative to salinity predicted by two end-members between freshwater and coastal water showed conservative mixing in AB. Silicate concentrations were significantly correlated with salinity and aCDOM(355) in AB in each month except for December 2014. These results suggest that silicate and CDOM in AB primarily originates from wetland-influenced river discharge. However, samples collected from the eelgrass meadows of AKE, where mariculture is developed, showed non-conservative mixing of silicate concentrations and aCDOM(355) with salinity except for June 2014. Elevated phosphate concentrations, probably released from sediments, were also found in the eelgrass meadows of AKE, especially during summer. These results suggest that the metabolic activities of mariculture and seagrass ecosystem significantly contribute to the nutrient cycles and CDOM absorption in AKE and to the distinct water-mass systems inside and outside AKE. The relative absorption properties of NAP [aNAP(443)], phytoplankton [aph(443)], and aCDOM(443) showed that CDOM is the main factor affecting the light distribution in AKE. However, the relative absorption properties varied seasonally in AB because of spring and autumn phytoplankton blooms and ice cover during winter. Significant relationships were observed between the Secchi disk depth (ZSD), aNAP(443), and aCDOM(443). Chl a concentration and aph(443) were not good indicators for predicting ZSD in our study region. These results suggest that incorporating inherent optical properties and CDOM from mariculture and seagrass ecosystem into ecosystem models could improve predictions of light distribution along the freshwater–eelgrass–coastal waters continuum in optically complex coastal waters.

Impact of a Large Shallow Semi‐Enclosed Lagoon on Freshwater Exchange Across an Inlet Channel

“…River flow is prescribed at the northern boundary with a vertically uniform southward velocity of 0.18 m s −1 and a width of 100 m. The discharge rate (Q) is 18 m 3 s −1 and the salinity (S) values of the incoming water and ambient oceanic water are 22 and 32 PSU, respectively, which are similar to observations (Ding, 2020;Isada et al, 2021). The salinity of the incoming water is non-zero, because the narrow pathway in the model is too short to reproduce mixing that takes place further upstream.…”

“…The inlet channel is 5 m deep on average and approximately 400 m wide at its narrowest point, representing the case of a shallow semi‐enclosed lagoon with a narrow inlet channel. The river discharge rate is approximately 18 m 3 s −1 (Ding, 2020), with a salinity of approximately 22 PSU near the river mouth (Isada et al., 2021). Freshwater flows out through the lagoon and channel to enter Akkeshi Bay, where it interacts with the Oyashio Current, the western boundary current of the North Pacific subpolar gyre.…”

Distribution of Harmful Algae (Karenia spp.) in October 2021 Off Southeast Hokkaido, Japan

“…Calculations of RBD and MLH were both based on the 665-753-nm spectral band, as opposed to OC4ME, which is based on the 443-560-nm spectral band. The 665-753-nm spectral band has an important advantage over the spectral band used in OC4ME in that the former is not strongly influenced by the absorption associated with colored dissolved organic matter (e.g., at around 440 nm), which is frequently found in the coastal waters of our study area (e.g., Isada et al, 2021).…”

Tidal effect on nitrate and dissolved organic carbon concentrations in river water in the Bekanbeushi River watershed in eastern Hokkaido, northern Japan

“…The study was conducted in the Bekanbeushi River watershed located in eastern Hokkaido, northern Japan (Supplemental Fig. 1) (Hayakawa et al 2006;Nagao et al 2016;Isada et al 2021). Most of the watershed is covered with cool-temperate mixed coniferous forest, secondary broadleaf forest, and forest plantation (Larix kaempferi, Abies sachalinensis, etc.…”

“…The lower reaches of the Bekanbeushi River are characterized by a tidal zone where the river level uctuates under the in uence of tides (Isada et al 2021), and the Paci c coast of eastern Hokkaido, where this river is located, is characterized by greater tidal uctuations than the Sea of Japan side in the west (Japan Meteorological Agency; https://www.data.jma.go.jp/gmd/kaiyou/shindan/index_tide.html). The microtidal ranges during the spring and neap tides are approximately 1.2 and 0.7 m, respectively (Isada et al 2021). The observation site is located in the lower reaches of the main river, where the river width is approximately 90 m and the depth at the river center varies in the range of approximately 1 to 2 m. Observations were made from mid-to late June 2022.…”