Chlorophyll Response to Shelf-Break Upwelling and Winds in the Chukchi Sea, Alaska, in Autumn

In the Chukchi Sea, autumn 1996 was windier than most of the previous 35 years. Conditions in the Bering Strait were anomalous, with fresh coastal water absent from the Strait and a partial flow reversal apparently occurring. In the central Chukchi Sea, the northeastward flow of the Alaskan Coastal Current was reversed. In the northern Chukchi Sea, upwelling of offshore, high-nutrient upper halocline waters inundated much of the shelf near Barrow Canyon. The resulting chlorophyll blooms indicate dramatically enhanced ecosystem productivity along this northern shelf region. With increasing climatic change occurring in this region, shelf break productivity would likely increase in the future. INTRODUCTION Approximately 0.8 Sv (1 Sv = 106 m3/s) of sea water passes northward through the Bering Strait, crosses the Chukchi Sea continental shelf, and enters into the Canadian Basin and Beaufort Seas [1]. The flow through the Strait is composed of two water types. Those upwelled near the Gulf of Anadyr in the northern Bering Sea are salty and nutrient rich, and those flowing northward along the Alaskan coast are strongly influenced by the Yukon River [2, 3]. As a result of the high nutrient concentrations and the intense mixing of waters within the Strait, algal productivity in the region north of the Strait is high (300 g C m-2 y-1 [4-6]). Because the shelf is wide (approximately 800 km from the Strait to the 200 m isobath) and the area influenced by the Bering Strait is large (300, 000 km2), the average volumetric residence time on the shelf is about 1 y [1, 7], although the pathways and transit times of the Bering Strait water types differ. The remnants of the denser Anadyr water sink and follow the longer topographic depression (Hope Valley and Herald Canyon) northwest from Bering Strait, potentially exiting the shelf near Wrangel Island [1]. A middle pathway between Herald Canyon and the Alaskan Coastal Current has been recently proposed [8, 9], but the flow in this pathway is weak and may simply reflect the overall net northward drift [7, 10]. With a transit time of several months in summer, the Alaskan Coastal Current follows the northwest Alaskan coast and enters the offshore Canada Basin in the vicinity of Point Barrow [1, 11]. By the time the Alaskan waters reach the Barrow region, inorganic nitrogen nutrients have disappeared from the water column and plant productivity is thought to be very low (40 g C m-2 y-1 [12-14]). This normal circulation may be interrupted by the strong northeasterly winds occurring in the autumn, and three char*Address correspondence to this author at the Bigelow Laboratory for Ocean Sciences, 180 McKown Point, West Boothbay Harbor ME 04575 USA; E-mail: [email protected] acteristic physical responses have been observed. Winds blowing south near the Bering Strait may cause partial flow reversals in the Strait and may alter the circulation south of the Strait [2, 15-18]. Along the northwest Alaskan coast, winds blowing from the northeast may block and reverse the Alaskan Coastal Current [11, 19, 20]. Within Barrow Canyon, upwelling of deeper offshore water may partially correlate with these same winds [11, 20, 21]. Additional upwelling has been noted east of Barrow Canyon, on the narrow shelves of the Beaufort Sea and eastern portion of the Chukchi Sea [22, 23]. Most reports of northern Alaskan shelfslope upwelling have focused on physical and hydrographic issues, but no study has examined the impact these events have on productivity. In an oligotrophic ecosystem, upwelling of nutrient rich water would be expected to measurably enhance productivity and abundances. Although upwelling has been mentioned as a possible cause of potentially richer euphotic zone conditions [14, 24-26], most recent studies have either occurred in the less-windy summer season or were conducted with sampling strategies that were too infrequent in space and time to evaluate details of possible upwelling events [13, 14, 24, 26, 27]. In this report, we examine the hydrographic, nutrient, and chlorophyll conditions found in the Chukchi Sea in the early fall of 1996. We first show that winds during this period were stronger than during most of the years of 1960-1995, so that affects of these winds would be expected to be more intense than normally found. Secondly we show evidence that the three hydrographic conditions associated with high winds did occur. Finally, we show that intense nutrient upwelling dramatically altered the inventory and distribution of chlorophyll in the northern Alaskan marine ecosystem. MATERIALS AND METHODS Hydrographic, nutrient, and chlorophyll measurements, acoustic doupler current profiler (ADCP) current measureChlorophyll Response to Shelf-Break Upwelling and Winds The Open Oceanography Journal, 2008, Volume 2 35 ments, and meteorological observations were made in the Chukchi Sea from the R.V. Alpha Helix between August 31 September 28 (Julian days 244-272), 1996 (Fig. 1). Individual stations were located along transect lines. Where transect lines were repeated, the line numbers are distinguished by the suffixes, a and b, for the first and second samplings, respectively. In order to characterize the relative intensity of the winds during the time of this cruise, long-term average winds were calculated from hourly Point Barrow airport data (Solar and Meteorological Surface Observation Network, 1961-1990, Hourly United States Weather Observations, 1990-1995, and hourly data from 1996). Upwelling intensity was evaluated based on the Bakun index [28], which represents the depthintegrated Ekman transport (m3 s-1 (m of coastline)-1) perpendicular to an ideal wind oriented in the same direction as the coastline. Since shipboard operations focused on the region just west of Point Barrow, the Bakun index [28] used the average orientation of the coast line between Point Barrow and Cape Lisburne (241°true). For the long-term Barrow data, averages over every 0 to 24 hour period within each year were calculated. The thirty-five year mean for each Julian day was then computed. Additional weather data were collected on the R/V Alpha Helix at approximately 2 minute intervals. For these data and the 1996 Barrow hourly data, four-hour averages were calculated and the Bakun index computed. During shipboard operations, over 200 continuous profiles of conductivity, temperature, pressure, light transmission, in situ fluorescence, and oxygen were made from the sea surface to the seafloor using the ship's Seabird CTD and Rosette system (Fig. 1). The downcast data was averaged over 1 dbar intervals, and Niskin bottles were tripped on the upcast. Salinities from bottle samples deeper than 100 m Fig. (1). Locations of CTD casts made in the Chukchi Sea between Julian days 244-272 (August 31-September 28), 1996. Small circles represent stations with CTD sensors only. Large circles are stations with additional water samples for nutrients and chlorophyll. Cast numbers are next to the circles. -172 -170 -168 -166 -164 -162 -160 -158 -156 -154 -152 64 65 66 67 68 69 70 71 72 73 74 Norton Sound Cape Prince of Wales Point Barrow Cape Lisburne 164 156