Variability of Primary Productivity
Below is a figure that illustrates monthly chlorophyll-a climatologies published in a recent paper produced by the retrospective data analysis component of the Gulf of Alaska Project. The authors found that the variability in the chlorophyll-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. Click here to download the full paper.
Primary production was anomalously low in 2011, so the retrospective data analysis team examined the chlorophyll-a anomaly with respect to the mean chlorophyll-a time series for 2002 – 2010 for the western and eastern Gulf of Alaska (divided at 145° W longitude). The figure below illustrates that in 2011 the two regions displayed markedly different patterns. The pattern in the western Gulf of Alaska was similar to previous years, though the spring and fall blooms began and ended earlier than usual. The apparent early spring bloom is due primarily to unusually high chlorophyll concentrations in the off-shelf waters. Concentrations on the shelf were slightly lower than usual, compared to the 2002 – 2010 mean.
Chlorophyll-a concentrations in the eastern Gulf of Alaska were low throughout the year; however, the biggest feature was what appears to be the lack of a spring bloom (see the figure below). 2011 appeared as an obvious outlier, both in terms of environmental factors and chlorophyll patterns. In particular, 2011 was characterized by a high Southern Oscillation Index (SOI), low Pacific Decadal Oscillation (PDO), high winter freshwater discharge, low spring freshwater discharge, and low upwelling, as well as higher than normal springtime chlorophyll-a concentrations in the western offshelf region, and lower than normal concentrations in the eastern and western shelf regions.
Larval Fish Life Cycles
The retrospective analysis group also investigated the typical phenology of larval fish in the Gulf of Alaska. The following table illustrates the timing of production and peak abundance of larvae relative to zooplankton production cycles and associated availability of larval prey. This information is will allow the ecological modeling team to initialize their models and will allow comparison of field data collected by this program to long-term patterns.
A manuscript is being developed (Doyle, Coyle and others) that will incorporate this seasonal schematic and investigate the early life history phenologies further with respect to detailed information on seasonality in the Gulf of Alaska zooplankton.
In 2011, field biologists caught very few young of the year fish that they were looking for offshore. The figure below illustrates the number of fish caught in the Gulf of Alaska in 2011. Summer Catches of age-0 arrowtooth flounder, rockfish, Pacific cod, sablefish, and pollock were low in 2011 and preliminary analyses indicate that the body sizes of both larval and juvenile fish were atypically small.
Fish sampling was only conducted in offshore waters during summer in 2012 and spring, summer, and fall in 2013. The highest fish biomass levels were located on the shelf seaward of Cross Sound and Chichagof Island in the eastern study region and over the shelf between Harris Bay and Stevenson Entrance in the western study region (see figures below). Age-0 rockfish and pollock were abundant in both study regions with the largest densities of rockfish encountered to date in slope and offshore habitats off Kodiak Island. Age-0 arrowtooth founder were abundant in the eastern region, but not in the western region.
Below are the results of acoustic surveys for fish and zooplankton collected in nearshore bays in the Gulf of Alaska in 2010 and 2011. Data were collected during spring, summer, and fall in 2013 and preliminary results will be available soon.
Click the images to expand.