Spatial and temporal variability of chlorophyll-a concentrations in the coastal Gulf of Alaska, 1998–2011, using cloud-free reconstructions of SeaWiFS and MODIS-Aqua data Jason N. Waite ⇑ , Franz J. Mueter University of Alaska Fairbanks, School of Fisheries and Ocean Sciences, Fisheries Division, 17101 Point Lena Loop Road, Juneau, AK 99801, USA article info Article history: Received 16 January 2013 Received in revised form 10 July 2013 Accepted 10 July 2013 Available online 20 July 2013 abstract We examined the spatial and temporal variability of chlorophyll-a (chl-a) concentrations in the coastal Gulf of Alaska (GOA) using a 14-year time series of cloud-free reconstructions of SeaWiFS and MODIS- Aqua chl-a data. Coast-wide and regional relationships between chl-a and likely environmental drivers, including anomalies in sea surface temperature (SST), photosynthetically-available radiation (PAR), sea surface height (SSHa), freshwater discharge, and coastal upwelling were explored. Coast-wide chl-a showed strong seasonal variability with pronounced spring and fall blooms, but both the magnitude and the seasonal patterns showed considerable interannual variability. Coast-wide annual chl-a anoma- lies were positive in years with elevated PAR, low SST, and a reduction in downwelling-favorable winds — conditions that are indicative of reduced cyclonic circulation associated with a weak Aleutian Low. The apparent negative effect of strong cyclonic circulation in the GOA on chl-a levels was further supported by negative relationships between elevated sea levels and coast-wide chl-a in both the spring and fall. If chl-a concentrations in the coastal GOA reflect productivity, these results are contrary to the prevailing view that strong cyclonic circulation enhances productivity in the GOA. The variability in the chl-a in each of four distinct and spatially contiguous regions that differed in the timing and magnitude of the spring and fall blooms was associated with different combinations of environmental variables. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The continental shelf and slope region of the Gulf of Alaska (GOA, Fig. 1) is a highly productive coastal region and supports a number of commercially important fisheries, in spite of being dom- inated by downwelling-favorable winds. Mechanisms supporting this high productivity include (1) onshore fluxes of nutrients at depth during periods when downwelling relaxes or when wind reversals lead to upwelling (Childers et al., 2005; Weingartner et al., 2009), coupled with episodic mixing due to tides and winds (Stabeno et al., 2004); (2) cross-shelf exchanges associated with eddies propagating along the slope, transporting macronutrients (e.g. nitrate, silicate) onto the shelf and supplying micronutrients (iron) to the slope and offshore regions (Childers et al., 2005; Her- mann et al., 2009; Janout et al., 2009; Ladd, 2005; Stabeno et al., 2004); (3) the interaction of the Alaska Current and Alaskan Stream with gullies and banks, resulting in onshelf fluxes of nutrient-rich slope waters (Stabeno et al., 2004); (4) vertical nutrient transport over shallow banks associated with tidal mixing (Hermann et al., 2009; Stabeno et al., 2004); and (5) upwelling events associated with cross-shelf shear resulting from local wind-stress curl such as that associated with barrier jets (Stabeno et al., 2004). How these and other mechanisms interact to determine patterns of pro- ductivity on the GOA shelf, from primary producers to upper tro- phic levels, remains poorly understood. The bathymetry of the coastal GOA is characterized by a 5– 200 km wide continental shelf that separates a coastal mountain range from abyssal depths in excess of 3000 m. The shelf has an ex- tremely irregular topography with shallow banks that are sepa- rated by numerous troughs or gullies cutting across the shelf. In Southeast Alaska, a narrow continental shelf is connected to the in- land waters of the Alexander Archipelago, which includes several deep straits and numerous estuaries. The shelf widens consider- ably in the western GOA and is connected to several large inlets, including Prince William Sound and Cook Inlet. Circulation in the coastal GOA is dominated by two major cur- rent systems: the subarctic gyre and the Alaska Coastal Current (Reed and Schumacher, 1986). The eastward-flowing North Pacific Current of the subarctic gyre bifurcates into two broad eastern boundary currents offshore of British Columbia. The Alaska Current is the northern extension of this current and flows northward along the coast of Southeast Alaska before turning westward along the continental slope off the southcentral coast of Alaska. It contin- ues as the Alaskan Stream along the south side of the Alaska 0079-6611/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pocean.2013.07.006 ⇑ Corresponding author. Tel.: +1 9077965455. E-mail addresses: jason.waite@alaska.edu (J.N. Waite), fmueter@alaska.edu (F.J. Mueter). Progress in Oceanography 116 (2013) 179–192 Contents lists available at ScienceDirect Progress in Oceanography journal homepage: www.elsevier.com/locate/pocean