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.
Highlights
Coastal oceans account for approximately 7–10% of the ocean’s surface, but contribute significantly to biogeochemical cycles (Walsh, 1991; Borges, 2005; Muller-Karger et al, 2005)
Ternary plots of absorption components showed that colored dissolved organic matter (CDOM) absorption was the main factor affecting light distribution at the surface of Akkeshi-ko estuary (AKE) across all seasons (Figure 7). These results suggest that the water mass in AKE was affected by CDOMrich wetland river discharge and the release of CDOM from eelgrass meadows and mariculture. aNAP(443), which reflects the presence of suspended particles, had a small impact on the light absorption budget in AKE, except in December 2014 because development of the eelgrass canopy contributed to a reduction in current velocity and a corresponding decrease in sediment resuspension in eelgrass meadows (Hasegawa et al, 2008)
This study is the first to investigate the seasonal and spatial distribution of the absorption properties of CDOM, non-algal particles (NAP), and phytoplankton as a function of temperature, salinity, nutrients, and water transparency along the river–eelgrass meadows– coastal waters continuum in the semi-enclosed coastal sea of Akkeshi-ko estuary and Akkeshi Bay, Japan, which includes the second largest Ramsar protected wetland in Japan
Summary
Coastal oceans account for approximately 7–10% of the ocean’s surface, but contribute significantly to biogeochemical cycles (Walsh, 1991; Borges, 2005; Muller-Karger et al, 2005). Nutrients and particulate and dissolved organic matter (DOM) are transported from the land to the ocean through rivers (Blough et al, 1993; Hedges et al, 1997; Canuel and Hardison, 2016). Silicate chemical weathering is one of the most importance process for transporting dissolved silicate from land to coastal areas via rivers (Tréguer et al, 1995; Tréguer and De La Rocha, 2013). The optical absorption properties of CDOM [aCDOM(λ)] can be used to identify different sources of DOM and dissolved organic carbon (DOC) along estuarine salinity gradients and are a valuable tool for tracing the influence of river discharge on coastal ecosystems (Coble, 2007; Stedmon and Nelson, 2014)
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