The amphipod Gondogeneia antarctica is among the most abundant benthic organisms, and a key food web species along the rapidly warming West Antarctic Peninsula (WAP). However, little is known about its trophic strategy for dealing with the extreme seasonality of Antarctic marine primary production. This study, using trophic markers, for the first time investigated seasonal dietary shifts of G. antarctica in a WAP fjord. We analyzed δ13C and δ15N in G. antarctica and its potential food sources. The isotopic signatures revealed a substantial contribution of red algae to the amphipod diet and also indicated a significant contribution of benthic diatoms. The isotope results were further supported by fatty acid (FA) analysis, which showed high similarities in FA composition (64% spring–summer, 58% fall–winter) between G. antarctica and the red algal species. G. antarctica δ13C showed a small shift seasonally (−18.9 to −21.4‰), suggesting that the main diets do not change much year-round. However, the relatively high δ15N values as for primary consumers indicated additional dietary sources such as animal parts. Interestingly, G. antarctica and its potential food sources were significantly enriched with δ15N during the fall–winter season, presumably through a degradation process, suggesting that G. antarctica consumes a substantial portion of its diets in the form of detritus. Overall, the results revealed that G. antarctica relies primarily on food sources derived from benthic primary producers throughout much of the year. Thus, G. antarctica is unlikely very affected by seasonal Antarctic primary production, and this strategy seems to have allowed them to adapt to shallow Antarctic nearshore waters.
Abstract. Small phytoplankton are anticipated to be more important in a recently warming and freshening ocean condition. However, little information on the contribution of small phytoplankton to overall phytoplankton production is currently available in the Amundsen Sea. To determine the contributions of small phytoplankton to total biomass and primary production, carbon and nitrogen uptake rates of total and small phytoplankton were obtained from 12 productivity stations in the Amundsen Sea. The daily carbon uptake rates of total phytoplankton averaged in this study were 0.42 g C m−2 d−1 (SD = ± 0.30 g C m−2 d−1) and 0.84 g C m−2 d−1 (SD = ± 0.18 g C m−2 d−1) for non-polynya and polynya regions, respectively, whereas the daily total nitrogen (nitrate and ammonium) uptake rates were 0.12 g N m−2 d−1 (SD = ± 0.09 g N m−2 d−1) and 0.21 g N m−2 d−1 (SD = ± 0.11 g N m−2 d−1), respectively, for non-polynya and polynya regions, all of which were within the ranges reported previously. Small phytoplankton contributed 26.9 and 27.7 % to the total carbon and nitrogen uptake rates of phytoplankton in this study, respectively, which were relatively higher than the chlorophyll a contribution (19.4 %) of small phytoplankton. For a comparison of different regions, the contributions for chlorophyll a concentration and primary production of small phytoplankton averaged from all the non-polynya stations were 42.4 and 50.8 %, which were significantly higher than those (7.9 and 14.9 %, respectively) in the polynya region. A strong negative correlation (r2 = 0. 790, p<0. 05) was found between the contributions of small phytoplankton and the total daily primary production of phytoplankton in this study. This finding implies that daily primary production decreases as small phytoplankton contribution increases, which is mainly due to the lower carbon uptake rate of small phytoplankton than large phytoplankton.
Antarctic shelf systems play an important role in organic matter circulation on Earth; hence, identifying the characteristics of dissolved organic matter (DOM) can be a good indicator for understanding its origin, as well as climate change. In this study, to identify the characteristics of DOM in the ice shelf systems, surface water was collected from the open sea (OS) and Marian cove (Fjord; FJ). Although there were no differences in DOM characteristics between sampling sites in quantitative analyses, the DOM in surface water of each region seemed to be more affected by terrestrial than marine biological sources in optical and molecular properties. This finding indicates that the terrestrial DOM related to mosses based on the molecular properties results; high levels of lipid-like (35–39%) and unsaturated hydrocarbon-like (UH; 27–34%) in both the OS and FJ regions, and significantly higher tannin-like substance and condensed aromatic structures (CAS) in the FJ than the OS region. When comparing the FJ transect samples, those nearest to a glacier (FJ1; 0.93 km from the glacier) showed relatively low salinity, high dissolved organic carbon (DOC), and high chromophoric DOM (CDOM), indicating that terrestrial DOM (possibly produced by moss) inflow occurred with the runoff from the freshly melting land ice and glacier. However, no significant differences in molecular composition were detected, suggesting that terrestrial DOM introduced into the ice shelf systems by melting land ice and glacier runoff could be a major source of DOM-rich seawater during austral fall, when low marine biological activity occurs. This study has a great significance as background data for DOM characteristics in the ice shelf systems due to the enhanced biological activity during the austral summer.
The spatio-temporal distributions of the epipelagic mesozooplankton community in the western Ross Sea region marine protected area (RSR MPA) were investigated. Mesozooplankton surveys were conducted in February 2018, January 2019, and March 2020 from an approximate depth of 200 m to address the essential environmental factors influencing the mesozooplankton community structure. Our results showed that the mesozooplankton community of the western RSR MPA could be affected by the various ecological factors, depending on their temporal and spatial variations. The community structure in 2018 was distinguished by its chlorophyll- a (Chl- a ) concentration during the summer bloom phase in the late summer. Taxa observed in 2019 were divided into four significantly different groups according to the body size of the community composition. This differentiation could be derived from predation pressure, inducing a trophic cascade. Taxa in the 2020 samples were separated into five different groups based on temperature; during the 2020 survey, the water temperature was low and sea ice covered the whole continental shelf in the Ross Sea. Additionally, comparing the results from the three interannual surveys, although the communities clustered according to the survey period, the continental shelf groups were quite dissimilar despite overlapping geographically. Taken all together, the mesozooplankton community of the western RSR MPA changed according to changes in several ecological factors, such as temperature, Chl- a concentration, and predation pressure. The occurrence of summer blooms and the decline in water temperature mainly regulated the mesozooplankton community structure in the late summer.
Abstract Atlantic‐origin cold saline water has previously not been considered an important contributor to the nutrient supply in the Pacific Arctic due to the effective insulation by the overlying Pacific‐origin waters that separate the surface mixed layer from the deeper Atlantic Water. Based on hydrographic observations in the northwestern Chukchi Sea from 2015 to 2017, we demonstrate that the intrusion of Atlantic‐origin cold saline water into the halocline boundary between Pacific and Atlantic‐origin waters in 2017 lifted Pacific‐origin nutrients up to the surface layer. We find that the cyclonic atmospheric circulation in 2017 was considerably strengthened, leading to lateral intrusions of two bodies of cold halocline water from the Eurasian marginal seas into the northwestern Chukchi Sea. Our results reveal that the intrusions of cold halocline waters caused unprecedented shoaling of the nutricline and anomalously high surface phytoplankton blooms in typically highly oligotrophic surface waters in the region during summer.
Dimethyl sulfide (DMS) production in the northern Arctic Ocean has been considered to be minimal because of high sea ice concentration and extremely low productivity. However, we found DMS concentration (1-33 nM) in melt ponds on sea ice at a very high latitude (78°N) in the central Arctic Ocean to be up to ten times that in the adjacent open ocean (<3 nM). We divided melt ponds into three categories: freshwater melt ponds, brackish melt ponds, and open saline melt ponds. Melt ponds from each category had different formation mechanisms and associated DMS contents. Closed brackish ponds (salinity of >20) had particularly high DMS concentration. Water in brackish ponds was mixed with open ocean water in the past via a hole at the bottom of the floe that kept the pond open to the ocean; therefore, unlike freshwater melt ponds, brackish ponds became sites of DMS accumulation. Our results suggest that continuous increase in melt pond coverage on Arctic sea ice could considerably impact future Arctic climate as well as enhancing DMS concentration in the Arctic atmosphere.
Nearshore fjords in the West Antarctic Peninsula (WAP) are experiencing rapid glacier retreat due to climate change. These areas host unique and diverse marine benthic inhabitants and are recognized as among the most vulnerable areas on Earth. However, knowledge on fjord benthic communities, particularly at the taxon level, is still very limited. Polychaetes are among the most abundant and specious fauna in the Southern Ocean. They also exhibit trophic diversities, likely serving as integral members of food webs. In this study, we investigated polychaete assemblages and their trophic interactions in Marian Cove (MC), a tributary fjord of Maxwell Bay (MB) in the WAP. We collected polychaete samples from several stations at varying distances from the glacier front using box corers and bottom trawls at depths ranging from 77 to 430 m and by SCUBA diving in shallower areas. We also utilized underwater video images as supplementary data. Our study revealed the presence of at least 41 polychaete species, showing assemblages comparable to those of other Antarctic fjords. The polychaete density and species number decreased toward the inner cove surrounded by retreating glaciers. The innermost sites showed a decrease in infaunal polychaetes, while filter-feeding epifaunal serpulids and sabellids were densely populated on hard substrates. Conversely, the outer cove exhibited a variety of taxa, with Maldanidae (53% of the total number) being the most abundant, followed by Terebellidae (14%), Cirratulidae (10%), Lumbrineridae (5%), and Ampharetidae (3%, mainly represented by the species Amythas membranifera). The overall polychaete density in the outer cove was several times lower (a mean of 21 inds·0.04 m-2) than that in a remote deeper MB station (92 inds·0.04 m-2) outside the MC, apparently due to low pelagic production and physical disturbances linked to glacier retreat. Despite the lower density, the outer cove showed the number of species similar to that at the MB station with more diverse feeding guilds from filter feeders to surface and subsurface deposit feeders. Additionally, isotopic analysis revealed that the major trophic groups utilize benthic primary producers as the main diet sources regardless of their feeding modes and habitat depths, indicating a crucial role of benthic production in maintaining the polychaete diversity. Our results reveal that polychaete communities in the fjord varied spatially in response to glacial processes, showing diverse and unique assemblages in some areas. Such diversity may be attributed to habitat complexity formed by glacial processes, to the confined nature of the MC, and, in part, to efficient utilization of benthic food sources.
Abstract Numerous studies have used stable isotope analysis (SIA) of carbon and nitrogen within the bulk tissues of organisms to determine the trophic structure among organisms in a food web. Recently, SIA has evolved to compound‐specific stable isotope analysis (CSIA) of nitrogen within amino acids to significantly reduce the uncertainty in the estimated trophic position (TP) of organisms based on the isotopic difference between glutamic acid and phenylalanine within a single organism. However, because the initial offset ( β ) between glutamic acid and phenylalanine differs between aquatic algae (ca. +3.4‰) and vascular plants (ca. −8.4‰) in food webs that rely on both resources, β should be replaced by a value adapted to the admixture of primary producers for each specimen. In this study, we established a new method involving the β value ( β mix ) of each consumer specimen determined based on its bulk tissue δ 13 C value and successfully obtained realistic TPs (TP mix ) for organisms in a complex seagrass meadow food web. Remarkable differences between the TP mix and traditional TP algal values were found in deposit feeders due to the large contribution of seagrass to their basal resources. The estimated TPs of organisms increased by up to 1.5 units (from TP algal to TP mix ), in terms of trophic transfer, when their diets included substantial seagrass‐derived contributions. Thus, combinatorial analysis of the amino acid δ 15 N and specimen‐specific β mix values provides better understanding of the trophic interactions in food webs, even in complex seagrass meadow ecosystems.
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