Annual Phytoplankton Production Research Articles

Seafloor primary production in a changing Arctic Ocean

Phytoplankton and sea ice algae are traditionally considered to be the main primary producers in the Arctic Ocean. In this Perspective, we explore the importance of benthic primary producers (BPPs) encompassing microalgae, macroalgae, and seagrasses, which represent a poorly quantified source of Arctic marine primary production. Despite scarce observations, models predict that BPPs are widespread, colonizing ~3 million km2 of the extensive Arctic coastal and shelf seas. Using a synthesis of published data and a novel model, we estimate that BPPs currently contribute ~77 Tg C y−1 of primary production to the Arctic, equivalent to ~20 to 35% of annual phytoplankton production. Macroalgae contribute ~43 Tg C y−1, seagrasses contribute ~23 Tg C y−1, and microalgae-dominated shelf habitats contribute ~11 to 16 Tg C y−1. Since 2003, the Arctic seafloor area exposed to sunlight has increased by ~47,000 km2 y−1, expanding the realm of BPPs in a warming Arctic. Increased macrophyte abundance and productivity is expected along Arctic coastlines with continued ocean warming and sea ice loss. However, microalgal benthic primary production has increased in only a few shelf regions despite substantial sea ice loss over the past 20 y, as higher solar irradiance in the ice-free ocean is counterbalanced by reduced water transparency. This suggests complex impacts of climate change on Arctic light availability and marine primary production. Despite significant knowledge gaps on Arctic BPPs, their widespread presence and obvious contribution to coastal and shelf ecosystem production call for further investigation and for their inclusion in Arctic ecosystem models and carbon budgets.

Estimates of prey requirements and initial examination of the primary productivity to support the magellan strait humpback whales

The waters of the Magellan Strait shelter a persistent and small feeding subpopulation of humpback whales (HWs) which is growing at a slower rate than expected, making it highly vulnerable. Despite their low abundance, the HWs represent significant consumers in this austral ecosystem. To better understand the needs of this summering subpopulation in the Magellan Strait feeding area, we estimated the biomass of prey required by HWs to meet their annual energy requirements using a velocity-dependent bioenergetic model and scaled it to two estimates of annual abundance (2011 and 2012), as well as the fraction of primary productivity required (PPR) to support that biomass of prey using a simple trophic energy transfer model. Based on a range of abundance between 75 and 101 whales, the summering subpopulation was estimated to require between 4,007 and 12,673 tons of prey under different population abundances estimates, energy needs, and residence times. These findings imply a minimum and maximum estimate ranging from 12.1 to 62.6 g C/m−2(−|-) y-1, or ∼ 6.8 to 41.5 % of net primary productivity in the feeding area to sustain the prey species directly required by HWs. Even though our estimates of prey requirements and PPR may change as new and better data become available, the results show that this small summering subpopulation requires at least a significant portion of annual phytoplankton production.

Annual New Production of Phytoplankton Estimated from MODIS-Derived Nitrate Concentration in the East/Japan Sea

Our main objective in this study was to determine the inter-annual variation of the annual new production in the East/Japan Sea (EJS), which was estimated from MODIS-aqua satellite-derived sea surface nitrate (SSN). The new production was extracted from northern (>40° N) and southern (>40° N) part of EJS based on Sub Polar Front (SPF). Based on the SSN concentrations derived from satellite data, we found that the annual new production (Mean ± S.D = 85.6 ± 10.1 g C m−2 year−1) in the northern part of the EJS was significantly higher (t-test, p < 0.01) than that of the southern part of the EJS (Mean ± S.D = 65.6 ± 3.9 g C m−2 year−1). Given the relationships between the new productions and sea surface temperature (SST) in this study, the new production could be more susceptible in the northern part than the southern part of the EJS under consistent SST warming. Since the new production estimated in this study is only based on the nitrate inputs into the euphotic depths during the winter, new productions from additional nitrate sources (e.g., the nitrate upward flux through the MLD and atmospheric deposition) should be considered for estimating the annual new production.

Estimation of Annual Kara Sea Primary Production

Primary production of phytoplankton and ice and under-ice flora of the Kara Sea and regions thereof has been assessed using region-specific models and MODIS-Aqua satellite data for 2002–2015. Average annual primary production of phytoplankton calculated for the growing season (April–October) amounted to 165 mg С m–2 day–1. Annual primary production of phytoplankton was 35 g C/m2. Annual primary production of phytoplankton in the entire Kara Sea was 13 × 1012 g C. Annual primary production of ice and underice flora calculated using an integrated biophysical model was 1.7 × 1012 g C, or 12% of total primary production of the Kara Sea; the ice cover dynamics and published data were taken into account for the calculations. The results have been compared to earlier primary production estimates for the Kara Sea. The extent of the increase in sea productivity during warming of the Arctic and the decrease in ice cover area are discussed.

Inter-Annual Variation of the Annual New Production of Phytoplankton in the Southwestern East/Japan Sea Estimated from Satellite-Derived Surface Nitrate Concentration

ABSTRACT Joo, H.; Lee, D.; Kang, J.J.; Lee, J.H.; Jeong, J.-Y.; Son, S.-H.; Kwon, J.-I., and Lee, S.H., 2018. Inter-annual variation of the annual new production of phytoplankton in the southwestern East/Japan Sea estimated from satellite-derived surface nitrate concentration. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 336–340. Coconut Creek (Florida), ISSN 0749-0208. The new production of phytoplankton based on nitrate is very important to understand how much portion of primary production passes on to higher trophic levels and sinks into deeper ocean. In this study, the inter-annual variation of the annual new production estimated from MODIS-aqua satellite-derived sea surface nitrate (SSN) concentration was analyzed in the southwestern East/Japan Sea from 2003 to 2015. Climatological SSN concentrations averaged in this study period ranged from 0.47 μM (±...

Oxygen and Nutrient Exchanges at the Sediment-Water Interface: a Global Synthesis and Critique of Estuarine and Coastal Data

Estuarine and coastal marine sediment-water fluxes are considered to be important ecological features, but a global-scale assessment has yet to be developed. Goals of this work were to assemble a global-scale database of net sediment-water flux measurements, examine measurement techniques, characterize the geographic distribution and magnitude of sediment fluxes, explore the data for controls on sediment flux magnitude, and assess the importance of sediment fluxes in ecosystem-level metabolism and primary production. We examined 480 peer-reviewed sources and found sediment flux data for 167 estuarine and coastal systems. Most measurements were made in North America, Europe, and Australia. Fluxes varied widely among systems, some by several orders of magnitude. Inter-annual variability within sites was less than an order of magnitude but time series flux data to evaluate this were rare. However, limited time series data exhibited large and rapid responses to decreased external nutrient loading rates, climate change effects (possible temperature effects), and variability in trophic conditions. Comparative analyses indicated organic matter supply to sediments set the upper limits of flux magnitude, with other factors playing secondary roles. Two metrics were developed to assess ecosystem-level importance of sediment-water fluxes. Sediments represented 30% or more of depth-integrated rates of aerobic system respiration at depths of <10 m. An annual phytoplankton production data set was used to estimate N and P demand; sediments supplied an average of 15–32% of N and 17–100% of P demand and, in some cases, was as large or larger than external nutrient inputs. The percent of demand supplied by sediments was highest in temperate latitudes and lower in high and tropical latitudes.

Autochthonous and allochthonous organic matter in trophic chain of lake ecosystems

This work presents analysis of mass–balance model simulating biotic flows of energy in pelagial of the large lakes of Russia (Ladoga, Onego, and Baikal) and small lake in northern part of the Karelia. The model had been developed on the basis of software package Stella and intended to predict an annual production of phytoplankton, bacterio plankton and consumers (non predatory and predatory zooplankton, planktivorous and piscivorous fishes). The input (independent) abiotic parameters of the model were latitude, mean lake depth, total phosphorus content and water color. The model analyzed an involvement of the autochthonous and allochthonous organic matter in the total energy flow through trophic chain. It was underlined that bacteria are important component of the planktonic community linking the dissolved organic matter (DOM) with organisms of the trophic chain. In high humic and oligotrophic boreal lakes the plankton respiration exceeds the primary production, therefore allochthonous DOM transformed to bacterial production replaces photosynthetic production in consumer feeding. The efficiency of bacterial growth (the ratio of bacterial production to consumed energy) depends on the ratio between autochthonous and allochthonous DOM. It had been shown that efficiency of bacterial growth in lakes with high primary production was higher than in oligotrophic waters with dominated allochthonous DOM. The author discussed different–type organic matter contribution to hydrobiont production depending on total phosphorus and humic matter content. Bacterioplankton consuming allochthonous DOM is additional energy source for zooplankton. For prediction of the total biological productivity it was recommended to take into account also production of heterotrophic bacterioplankton which is involved in utilization of allochthonous DOM.

Quantitative distribution and feeding of euphausiids in the Okhotsk Sea

Euphausia in the Okhotsk Sea is one of the most abundant groups of zooplankton, its portion in the total zooplankton biomass in the epipelagic layer is 25.4-37.4 %, depending on season, the total stock is 83.2 · 10 6 t in spring, 66.3 · 10 6 t in summer, 57.4 · 10 6 t in autumn, and 73.3 · 10 6 t in winter (mean assessments for 1986-2012). The highest biomass is observed in spring - 369 mg/m 3 , on average, and the minimum one in autumn - 256 mg/m 3 , on average. Two species, Thysanoessa raschii and Thysanoessa longipes , are the most abundant. Year-to-year fluctuations of Euphausia abundance are prominent: up to 5 times, but it never causes any catastrophic change in their populations. Sharp declines usually are followed by sharp rises in their number. Mean daily ration of euphausiid in the Okhotsk Sea is 7.5-8.5 % of the body weight. Annually they consume 2012.7 · 10 6 t of prey, including 1636.5 · 10 6 t of phytoplankton and 341.8 · 10 6 t of micro-heterotrophs. About 11.4 % of the annual production of phytoplankton and 26.0 % of the annual production of micro-heterotrophs are grazed by euphausiids.

Environmental characteristics and phytoplankton productivity of a shallow Ramsar‐site floodplain in the western Himalaya

Hokersar wetland (altitude of 1584 masl), a shallow (0.5 m) floodplain waterfowl habitat in Kashmir, India, gained international importance in 2005 with its declaration as a Ramsar site. Although isolated in the western Himalayan mountains, it is being impacted by a silt-laden river (Doodhganga). Human activities, not the least of which is affected by the closeness of the wetland to suburban areas, have gradually altered its trophic state. Its alkaline- and calcium-rich waters contain many planktonic diatoms, green algae and cyanobacteria. Nitrate–nitrogen and total phosphorus concentrations are high (370–4750 and 101–968 μg L−1, respectively). A pronounced seasonal cycle in phytoplankton production (g C m−2 day−1) was evident in a minimum value of 0.38 (Jan–Feb, 2002) and a maximum value of 4.02 (July, 2002), closely paralleling the seasonal cycles of temperature and light. The photosynthetic efficiency of the wetland was highest (1.97%) during the summer. The annual phytoplankton production of this wetland ecosystem was 210 × 102 KJ m−2, suggestive of a meso-eutrophic status.

Dreissenid metabolism and ecosystem-scale effects as revealed by oxygen consumption

Respiration rates of the quagga mussel (Dreissena rostriformis bugensis) were determined for the shallow and profunda morph phenotypes from in situ and laboratory experiments under a range of temperature (4–20°C), shell size, and food and oxygen availability conditions. Temperature-normalized oxygen consumption was significantly lower for the hypolimnetic profunda phenotype than for the shallow type. Mass-normalized respiration rates were inversely related to mussel size. Mussels adjusted their oxygen consumption in response to food enhancement and deprivation, lowering their respiration to a basal metabolic rate 18 hours after food deprivation. In response to decreasing ambient oxygen, quagga mussels exhibited first-order reaction kinetics, with mass-normalized respiration rate at a dissolved oxygen concentration of 0.002 mol L− 1 being 1% of that at saturation. Using published data on quagga mussel energy budgets and respiratory quotients, oxygen consumption rates were converted to organic carbon consumption rates. Using these values, along with data on mussel density and size frequency distribution, it is estimated that quagga mussels consume 54% of annual phytoplankton production, from 1.4–4.1 times the offshore annual settled organic carbon in the southern basin of Lake Michigan, and from 2.4–5.5% of offshore areal particulate carbon in the water column. Dreissenids appear to exert significant direct influences on benthic oxygen dynamics, and consume enough organic carbon to have a significant effect on energy flow in the Lake Michigan ecosystem.

The effects of changing winds and temperatures on the oceanography of the Ross Sea in the 21st century

AbstractThe Ross Sea is critically important in regulating Antarctic sea ice and is biologically productive, which makes changes in the region's physical environment of global concern. We examined the effects of projected changes in atmospheric temperatures and winds on aspects of the ocean circulation likely important to primary production using a high‐resolution sea ice‐ocean‐ice shelf model of the Ross Sea. The modeled summer sea‐ice concentrations decreased by 56% by 2050 and 78% by 2100. The duration of shallow mixed layers over the continental shelf increased by 8.5 and 19.2 days in 2050 and 2100, and the mean summer mixed layer depths decreased by 12 and 44%. These results suggest that the annual phytoplankton production in the future will increase and become more diatomaceous. Other components of the Ross Sea food web will likely be severely disrupted, creating significant but unpredictable impacts on the ocean's most pristine ecosystem.

Prediction and comparative characteristic of the biological production of reservoirs on the Zeya and Bureya Rivers

A mass balance model is applied to assess the biotic energy flux in ecosystems of the operating Zeiskoe and Bureiskoe reservoirs and the anticipated Lower Zeiskoe and Lower Bureiskoe Reservoirs. A forecast of the mean annual biomass and annual production of phytoplankton, macrophytes, epiphytes, and phytobenthos (primary producers), bacterioplankton, bacteriobenthos (decomposers), “peaceful” and predatory zooplankton, zoobenthos, plankton-feeding, benthos-feeding, and predatory fish (consumers).

Production of the eel grass Zostera marina Linnaeus, 1753 in the White Sea

The results of year-round studies of the photosynthesis and respiration rates of the eel grass Zostera marina L. in the White Sea are presented. The annual production of eel grass is estimated to be 9.86 × 1011 kcal. It is concluded that the total annual production of the eel grass constitutes about 3% of the annual phytoplankton production (3 × 1013 kcal; according to the data of Bobrov et al., 1995) in the White Sea.

Eutrophication and annual primary production of phytoplankton in the deep-water part of the Black Sea

Using the data of daily primary production, as well as intraannual and long-term changes in the concentration of chlorophyll “a” and hydrochemical characteristics, the annual primary production of phytoplankton in the deep-water part of the Black Sea is estimated for the three key periods in the contemporary evolution of the sea: preeutrophication, very intense eutrophication, and the present-day period characterized by deeutrophication. It is shown that eutrophication in the second part of the 20th Century led to an increase in the production level not only in the shelf of the Black Sea, but also its deep-water areas. By the end of the 1980s and the early 1990s, the value of the annual primary production in this part of the sea increased from 63 ± 18 g C m−2 yr−1 (in the 1960s) up to 135 ± 30 g C m−2 yr−1. On the contrary, after 1993, mainly because of reduced runoff of biogenic substances into the Black Sea from land based sources, there was a decrease in the annual production of phytoplankton in the deep-water areas of the sea, which is currently about 105 g C m−2 yr−1.

Detrital production in Nova Scotian kelp beds: patterns and processes

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 421:67-82 (2011) - DOI: https://doi.org/10.3354/meps08905 Detrital production in Nova Scotian kelp beds: patterns and processes Kira A. Krumhansl1,*, Robert E. Scheibling1 1Department of Biology, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada *Email: kira.krumhansl@dal.ca ABSTRACT: Connectivity via the transport of detrital material from areas of high to low productivity may be an important determinant of secondary productivity and biodiversity in receiving communities. On the Atlantic coast of Nova Scotia, detritus exported from subtidal kelp beds contributes to food webs in communities inhabiting deeper waters offshore. To estimate the amount of energy available via this pathway, we measured rates of detrital production via erosion of kelp blades in kelp beds at 5 sites varying in wave exposure. Specifically, we measured productivity and erosion of the 2 dominant species of kelp, Laminaria digitata and Saccharina longicruris, over 16 mo. We also examined the effect of environmental and biological factors on erosion rates, including temperature, wave exposure, grazing by snails and cover by the invasive bryozoan Membranipora membranacea. We observed clear seasonal and spatial patterns in productivity, with the highest production in spring. Erosion rate was highly variable across sites and seasons, and was strongly related to the cover of M. membranacea and the intensity of snail grazing at the distal end of kelp blades. Both factors cause tissue degradation, abrasions and perforations that weaken blades. Erosion rate was also positively related to water temperature and site exposure. Annual detrital production from the erosion of kelp blades increased linearly with kelp bed biomass, and ranged from 0.5 to 1.71 kg dry weight m–2 (150–513 g C m–2) across sites. These rates equal or exceed annual phytoplankton production off the Atlantic coast of Nova Scotia and estimates of detrital production in seagrass beds in North America. KEY WORDS: Detritus · Grazing · Invasive species · Kelp bed · Laminaria digitata · Membranipora membranacea · Productivity · Saccharina longicruris Full text in pdf format Supplementary material PreviousNextCite this article as: Krumhansl KA, Scheibling RE (2011) Detrital production in Nova Scotian kelp beds: patterns and processes. Mar Ecol Prog Ser 421:67-82. https://doi.org/10.3354/meps08905 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 421. Online publication date: January 17, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

The reflection of the present distribution of the primary production in the bottom sediments of the Sea of Okhotsk

Using the authors’ data obtained during expeditions to the Sea of Okhotsk during the last decade, the primary phytoplankton production and the distribution of organic carbon and chlorophyll a degradation products (chlorin) in the bottom sediments were studied. Using the authors’ and published data, the spatial distribution of the production and paleoproduction indicators was plotted. The ratios of the chlorin and Corg content in the sediments was considered, and the correlation between these parameters was revealed. It was shown that the average annual primary production of phytoplankton and the paleoproduction indices were maximum in the coastal and upwelling zones and decreased towards the sea’s center. A quantitative correlation was found between the distribution of the present primary production in the photic layer and that of the rates of the accumulation of organic matter buried within the surface sediments. As a result, it was shown that the content of chlorin and Corg in the marine bottom sediments may be used to reconstruct the paleoproduction variability of the past.

Seasonal dynamics and estimation of the annual primary production of phytoplankton in the Black Sea

Seasonal variations in the surface chlorophyll a concentrations (Chl s) and the integrated primary production (PP inf) were investigated for ten regions of the Black Sea based on long term observations (1973–1997). Two or three maximums of both Chl s and PP inf were registered in most of the shelf regions (SR, 1500 m) in February–March, June–August, and October–November. Such a pattern suggests that the seasonal dynamics of PP inf strongly depend on the Chl s variability. The mean annual values of the PP inf comprised 130–420, 130–150, and 140–150 g C m−2 in the SR, CS, and DSR, respectively. These values are mainly typical of the eutrophic layer and the transition between the eutrophic and mesotrophic waters (SR) or for the upper boundary of the mesotrophic waters (CS and DSR). The maximal contribution of the wintertime (December–March) to the total PP inf values (40–42%) was observed in the DSR. In the SR and the adjacent eastern CS areas, the proportion of the PP inf summertime production (June–September) reaches 40–60% and is higher than the wintertime production. The lowest values of PP inf (9–17%) were produced in the spring and autumn periods. The total annual values of PP inf in the Black Sea are close to 50–70 Mt C.

Chlorophyll, primary production, and organic carbon fluxes in the Kara Sea

New maps of the mean monthly distribution of chlorophyll and the primary production in the Kara Sea were compiled using joint processing of CZCS (1978–1986), SeaWiFS (1998–2005), and MODIS (2002–2006) satellite data and field measurements. The annual primary production of phytoplankton is estimated at 22.3 × 106 t of C per year or 70 mg of C/m2 per day. The results of the calculations of the organic carbon budget in the Kara Sea are presented.

Recent carbon and nitrogen uptake rates of phytoplankton in Bering Strait and the Chukchi Sea

Cruises to Bering Strait and the Chukchi Sea in US waters from late June in 2002 to early September in 2004 and the Russian–American Long-term Census of the Arctic (RUSALCA) research cruise in 2004 covered all major water masses and contributed to a better understanding of the regional physics, nutrient dynamics, and biological systems. The integrated concentration of the high nitrate pool in the central Chukchi Sea was greater in this study than in previous studies, although the highest nitrate concentration (∼22 μM) in the Anadyr Water mass passing through the western side of Bering Strait was consistent with prior observations. The chlorophyll- a concentrations near the western side of the Diomede Islands ranged from 200 to 400 mg chl- a m −2 and the range in the central Chukchi Sea was 200–500 mg chl- a m −2 for the 2002–2004 Alpha Helix (HX) cruises. Chlorophyll- a concentrations for the 2004 RUSALCA cruise were lower than those from previous studies. The mean annual primary production of phytoplankton from this study, using a 13C– 15N dual-isotope technique, was 55 g C m −2 for the whole Chukchi Sea and 145 g C m −2 for the plume of Anadyr–Bering Shelf Water in the central Chukchi Sea. In contrast, the averages of annual total nitrogen production were 13.9 g N m −2 (S.D.=±16.2 g N m −2) and 33.8 g N m −2 (S.D.=±14.1 g N m −2) for the Chukchi Sea and the plume, respectively. These carbon and nitrogen production rates of phytoplankton were consistently two-or three-fold lower than those from previous studies. We suggest that the lower rates in this study, and consequently more unused nitrate in the water column, were caused by lower phytoplankton biomass in the Bering Strait and the Chukchi Sea. However, we do not know if the lower rate of production from this study is a general decreasing trend or simply temporal variations in the Chukchi Sea, since temporal and geographical variations are substantially large and presently unpredictable.

Carbon cycling in a high-arctic marine ecosystem – Young Sound, NE Greenland

Young Sound is a deep-sill fjord in NE Greenland (74°N). Sea ice usually begins to form in late September and gains a thickness of ∼1.5 m topped with 0–40 cm of snow before breaking up in mid-July the following year. Primary production starts in spring when sea ice algae begin to flourish at the ice–water interface. Most biomass accumulation occurs in the lower parts of the sea ice, but sea ice algae are observed throughout the sea ice matrix. However, sea ice algal primary production in the fjord is low and often contributes only a few percent of the annual phytoplankton production. Following the break-up of ice, the immediate increase in light penetration to the water column causes a steep increase in pelagic primary production. Usually, the bloom lasts until August–September when nutrients begin to limit production in surface waters and sea ice starts to form. The grazer community, dominated by copepods, soon takes advantage of the increased phytoplankton production, and on an annual basis their carbon demand (7–11 g C m −2) is similar to phytoplankton production (6–10 g C m −2). Furthermore, the carbon demand of pelagic bacteria amounts to 7–12 g C m −2 yr −1. Thus, the carbon demand of the heterotrophic plankton is approximately twice the estimated pelagic primary production, illustrating the importance of advected carbon from the Greenland Sea and from land in fuelling the ecosystem. In the shallow parts of the fjord (<40 m) benthic primary producers dominate primary production. As a minimum estimate, a total of 41 g C m −2 yr −1 is fixed by primary production, of which phytoplankton contributes 15%, sea ice algae <1%, benthic macrophytes 62% and benthic microphytes 22%. A high and diverse benthic infauna dominated by polychaetes and bivalves exists in these shallow-water sediments (<40 m), which are colonized by benthic primary producers and in direct contact with the pelagic phytoplankton bloom. The annual benthic mineralization is 32 g C m −2 yr −1 of which megafauna accounts for 17%. In deeper waters benthic mineralization is 40% lower than in shallow waters and megafauna, primarily brittle stars, accounts for 27% of the benthic mineralization. The carbon that escapes degradation is permanently accumulated in the sediment, and for the locality investigated a rate of 7 g C m −2 yr −1 was determined. A group of walruses (up to 50 adult males) feed in the area in shallow waters (<40 m) during the short, productive, ice-free period, and they have been shown to be able to consume <3% of the standing stock of bivalves ( Hiatella arctica, Mya truncata and Serripes Groenlandicus), or half of the annual bivalve somatic production. Feeding at greater depths is negligible in comparison with their feeding in the bivalve-rich shallow waters.