Abstract
Abstract. We analysed mooring and ship-based hydrographic and biogeochemical data obtained from a Hope Valley biological hotspot in the southern Chukchi Sea. The moorings were deployed from 16 July 2012 to 19 July 2014, and data were captured during spring and autumn blooms with high chlorophyll a concentrations. Turbidity increased and dissolved oxygen decreased in the bottom water at the mooring site before the autumn bloom, suggesting an accumulation of particulate organic matter and its decomposition (nutrient regeneration) at the bottom. This event may have been a trigger for the autumn bloom at this site. The bloom was maintained for 1 month in 2012 and for 2 months in 2013. The maintenance mechanism for the autumn bloom was also studied by hydrographic and biogeochemical surveys in late summer to autumn 2012 and 2013. Nutrient-rich water from the Bering Sea supplied nutrients to Hope Valley, although a reduction in nutrients occurred in 2012 by the influence of lower-nutrient water that would have remained on the Chukchi Sea shelf. In addition, nutrient regeneration at the bottom of Hope Valley could have increased nutrient concentrations and explained 60 % of its nutrient content in the bottom water in the autumn of 2012. The high nutrient content with the dome-like structure of the bottom water may have maintained the high primary productivity via the vertical nutrient supply from the bottom water, which was likely caused by wind-induced mixing during the autumn bloom. Primary productivity was 0.3 g C m−2 d−1 in September 2012 and 1.6 g C m−2 d−1 in September 2013. The lower productivity in 2012 was related to strong stratification caused by the high fraction of surface sea ice meltwater.
Highlights
The southern Chukchi Sea is one of the most biologically productive regions of the world’s oceans because of nutrients supplied by northward flow of Pacific-originating water advected over the shelves from the northern Bering Sea into the Arctic Ocean (McRoy, 1993; Springer and McRoy, 1993; Hunt et al, 2013)
PAMW value of 793 μmol kg−1 was estimated by YamamotoKawai et al (2009), but this value was obtained from samples collected in the Canada Basin, which is farther north than the study area in the southern Chukchi Sea and under an apparent larger influence of Eurasian rivers
Because the study area is rather influenced by North American rivers, we assumed the PAMW to be 1620 μmol kg−1, based on the flow-weighted average of alkalinity estimated from the rivers (Cooper et al, 2008)
Summary
The southern Chukchi Sea is one of the most biologically productive regions of the world’s oceans because of nutrients supplied by northward flow of Pacific-originating water advected over the shelves from the northern Bering Sea into the Arctic Ocean (McRoy, 1993; Springer and McRoy, 1993; Hunt et al, 2013). Such a region of high biological activity is called a biological hotspot Including this southern Chukchi Sea biological hotspot, the international Distributed Biological Observatory (DBO, http: //www.arctic.noaa.gov/dbo/) designated five locations in the Pacific Arctic domain, spanning the latitudinal range from the northern Bering Sea to the northern Chukchi Sea, as important locations for ecosystem monitoring.
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