Nutrient fluxes during extended blooms of Arctic ice algae

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, NO. C2, PAGES 1951-1962,FEBRUARY 15, 1987

Nutrient Fluxes During Extended Blooms of Arctic Ice Algae G. F. COTA, TMS. J. PRINSENBERG, 2 E. B. BENNETT, 2 J. W. LODER, 2 M. R. LEWIS, 3 J. L. ANNING, N.H. F. WATSON, 1 AND L. R. HARRIS1 Estimatesof nutrient demandby densemats of ice algae in the high Arctic indicatethat substantial nutrientfluxesare necessary to satisfythe observedgrowth over the 2-monthbloom.In our studyarea, Barrow Strait, the quantity of nutrientsin the surface-mixedlayer is about 3-10 times greater than estimatesof total demand during the bloom, and nutrient fluxesin the water column are estimatedto be of the sameorder of magnitudeas algaldemand.The fluxesin the watercolumnare predictedto vary by morethan an orderof magnitudeoverthe fortnightlytidal cycle,assuming that fluxesdependuponthe strength of tidal currents and the vertical nutrient gradients.In the latter half of the bloom, when biomasslevels are high, it appearsthat establishedpopulationsof ice algae may experiencecyclic conditionsof nutrient limitation during neap tides when nutrient fluxesare minimal. Contributions from regenerationand brine exclusionfrom the ice sheetappearto satisfyonly a portion of the bloom'stotal requirementfor nutrients.

been ignored or consideredas unimportant. Becauseof the relatively high levelsof nutrients in polar waters,most investiDensegrowthsof ice algaeare a ubiquitousfeaturebeneath annual sea ice in polar regions during the springtime.In the gators have apparently assumedthat sufficient,if not luxurhigh Arctic thesemicroalgaeare largelyconcentratedat the iant, suppliesof nutrients were available for ice algal growth. ice-water interface on or withintheskeletal layerof thecon- However, simpleestimatesof nutrient requirements,basedon gelationice.MacroScopically, theyresemble a mottledgolden- cellular compositionand observedrates.of increasein plant brown carpet about 1-2 cm thick. We have found that these biomass,indicate that substantialnutrient fluxes are necessary algae grow for at least 2 months (April-May) in the central to sustain theseextendedblooms (i.e., 2 months) of ice algae. portion of the Northwest Passage.Moreover, the ice alga.e These estimates of nutrient demand allow us to place lower often achieve high levels of biomass(80-100 mg chlorophyll bounds on the fluxes required to satisfy algal growth and INTRODUCTION

checkon themagnitude of m-e) that are similarto watercolumnvaluesintegratedover provideuswithan independent thedepthoftheeuphotic zonein manyplanktonic systems. Of all the potentially limiting environmental factors, only light has received much attention. A number of investigators

havesuggestted thepresence ofa critica 1threshold forlightto explain the onset of the bloom [e.g., Clasby et al., 1973' Horner and Schrader, 1982' Gosselin et al., 1985]. However, during the latter half of the bloom (May), light is more or less constant under conditions of continuous day!ight, and the

other physicalestimatesof fluxes. Meguro et al. [1967] proposed three potentially important sourcesof nutrient supply for ice algae: in situ regenerationof inorganic nutrients from dissolved and particulate biogenic materials, nutrient flux from the water column, and nutrient

flux du.eto desalination (brinerejectionand drainage)of the overlying ice sheet.In this paper we examine the relative im-

portanceof thesenutrientsourcesto ice algae growth in

algaeare well adaptedto their light environment'[Cora,1.985' Barrow Strait, Northwest Territories, Canada, paying particuto thewatercolumnsupply. Thedatausedin this G. F. Cota and E. P. W. Horne, unpublished manuscript, lar attention

1986].Furthermore,watertemperature is virtuallyconstant paper were collectedduring a multiyearinterdisciplinary

throughoutthe bloom, and yet we have repeatedlyobserved oceanographicprogram in Barrow Strait which included inlarge fluctuations' in biomass, apparent growth rates, and photosynthesisrates in the Arctic and Antarctic [Cota and

vestigations of other aspects of the physical and biological

regimesI-Prinsenberg andBennett,1987;N. F. Watson,unpub-

lished manuscript, 1986; Conover et al., 1986]. We draw on data from severalfield seasonsbut concentrate on information Sullivan, unpublished data,1986]. With a few notable exceptions[Meguro et al., 1967; Goering collectedduring.the spring from April through mid-June for and McRoy, 1974' Grainget, 1977' Gossdinet al., 1985' Smith 1981-1984.EmplOyingsimplebiologicaland physicalmodels, et al. 1987-1,nutrient limitation of ice algal growth has usually we show that nutrient fluxes in the water column are, at least, of the same order of magnitude as nutrient demand by the ice algae, while other sourcesof supply accountfor only a portion of the minimum required nutrient. x Departmentof Fisheries and Oceans,MarineEcologyLabora-

Horne,unpublished manuscript, 1986'G. F. Cota and C. W.

tory, Bedford Institute of Oceanography,Dartmouth, Nova Scotia,

PHYSICAL AND CHEMICAL ENVIRONMENT

Canada.

2 Di•partment of Fisheries and Oceans,AtlanticOceanographic Barrow Strait is centrally located in the Northwest Passage (Parry Channel)throughthe CanadianArchipelago(Figures1 3 Department of Oceanography, Dalhousie University, Halifax, and 2a). Sourcewatersfor the regionare largelyArctic Ocean Nova Scotia, Canada. Laboratory,BedfordInstitute of Oceanography,Dartmouth, Nova

Scotia, Canada.

'• Now at Department of Biological Sciences, University of Southern California,LosAngeles. Publishedin 1987by the AmericanGeophysicalUnion. Paper number 6C0587.

surface water

Water 1951

which

enters Barrow

Strait

from

the west

through Viscount Melville Sound and from the north via Penny Strait around both sidesof Cornwallis Island I-LeBlond, 1980; Fissel and Marko, 1978; Prinsenbergand Bennett, 1987]. from

Peel Sound

also flows into

the south

side of

1952

COTA ET AL.: NUTRIENT FLUXES DURING EXTENDED BLOOtaSOF ARCTIC ICE ALGAE

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Fig. 1. The Canadian Arctic with an inset of the study region.

Barrow Strait; it is less saline and thus the lightest water entering the Barrow Strait region. Regional conductivity and temperature profile surveys (Figure 2b) show that the density structure varies both along and across(Figure 3) Barrow Strait [Prinsenbergand Sosnoski, 1983a, b]. Current structure was obtained with current meters and with a gyroscopic unmanned profiling system (GUMPS) deployed at stations 31, 42, and 46 (Figure 2b). Hourly profiles of current speed, direction, conductivity, temperature, and pressure were collected with the GUMPS at 11-14 discrete depths. The data show that tidal currents dominate the total

Figure 5 showsrepresentativenutrient profiles through the water column at stations 5, 3, and 42. Below the pycnocline (> 50 m) there is relatively little structure, and variability is low (< 10% of the mean concentration). Average nutrient concentrationsin the surface "mixed" layer vary by as much as 20-40% during the spring. Jones and Coote [1980] ob-

flow, which reachesspeedsof up to 60 cm s-• during spring

In the estimation of the plants' nutrient requirements we will concentrateon dissolvedsilicon as silicic acid (Si(OHh) and nitrogen in the form of nitrate (NO3) becausemost nitrogen is present as nitrate (Alexander et al. [1974], Grainger [1977], Horner and Schrader [1982], and this study). We

tides. The residual mean flows are generally eastward, in geo-

strophiebalance,and reachspeedsof up to 10-15 cm s- •. The cross-channelcurrents, representedby the arrows in Figure 3, indicate a general southward drift across the strait with a surface return flow on the south shore. Pycnoclines (Figure 3) tend to be sharper and more shallow on the northern side of Barrow Strait, while in Resolute Passage(station 31), often no stratification is observed(Figures 4a and 4b). Water from McDougall Sound (station 72) flows south through Resolute Passage (station 31), where surface layer mixing typically extends to 50 m but may reach 100 m during strong tidal currents (Figure 4c). Periodically, a small amount of dilution with the less saline water from the west (Viscount Melville Sound) occurs.

Ice growth is from the lower surfaceand is largely complete by late April when typically, 1-2 m of ice is present. Accretion of ice in April 1983 and 1984 at the main study sites(stations 42 and 3, Figure 2b) did not exceed 15 cm, and final ice thickness were between 1.7 and 1.9 m, respectively,for 1983 and 1984, which representrelatively heavy ice years. The large-scale features of the distributions of nutrient salts in the Arctic Ocean (upstream), the Canadian Archipelago, and Baffin Bay (downstream) are discussedby Kinney et al. [1970], Jones and Coote [1980], and Coote and Jones [1982]. Nutrient levels, except nitrate, in our study region are markedly higher (2-5 times) than the "arctic" waters (Barents Sea) consideredby Sakshaugand Holm-Hansen [1984].

served similar

vertical

structure

in their

summer

data

from

Barrow Strait except that near-surface values (