with warmth of light’s return
Ocean life renewed
Why are we sampling?
Plankton, the organisms that drift with ocean currents, are the base of every marine food chain. Both phytoplankton (plant) and zooplankton (animal) are sensitive to physical, chemical and biological changes in the environment and respond rapidly to a range of environmental stressors. Subtle changes in the plankton community, such as in the types and abundance of different plankton species, in the size range of plankton, in the timing of an annual plankton bloom, or in the total biomass of phytoplankton and zooplankton, can have profound effects on the species at higher levels in a marine food web. In the Gulf of Alaska, the higher-level species include marine mammals, seabirds, and salmon, as well as herring, sand lance, and capelin, known collectively as “forage fish” because they have so many predators. Many predators migrate between the area affected by the Exxon Valdez oil spill on the shallow Gulf of Alaska continental shelf and offshore waters. Some depend on prey that migrate between the two types of habitats. An understanding of the productivity of both the continental shelf and offshore areas is therefore important to our overall understanding and to predictions of changes in the abundance of animals throughout the oil spill-affected area.
The objective of the Continuous Plankton Recorder (CPR) project is to sample plankton from ships that make repeated transits across the Gulf of Alaska. These are called “vessels of opportunity” because they are not research vessels – they are cargo ships that carry CPR instrumentation. As a vessel of opportunity makes its transit, the CPR collects data about the abundance and biomass of different types of plankton continuously. Plankton data collected in this way in the Gulf of Alaska during spring and summer seasons since 2000 and the data have been summarized and analyzed on a monthly basis. These data have proved important in helping us understand factors that may have caused changes within the ecosystem at different times and within different regions.
Where are we sampling?
Plankton data are collected on a path that begins in the open Gulf of Alaska crosses the “shelf break” where the sea bottom steeps sharply upward to the shallower continental shelf, and then crosses the inner part of Cook Inlet.
How are we sampling?
The cargo vessel Matson Kodiak tows the Continuous Plankton Recorder (CPR, pictured) northbound towards Cook Inlet approximately once per month between April and September each year. The samples are unloaded and the gear serviced each time by Alaskan technicians who have been trained by the Marine Biological Association (MBA). Sample processing is carried out at the Department of Fisheries and Oceans laboratory in Sidney, British Columbia and at the MBA laboratory where the plankton, the majority of which are microscopic, are identified and counted.
What are we finding?
We are investigating the impacts of the Pacific marine heatwave of 2014-2016 on the lower trophic levels and assessing whether post-heatwave years changed to more typical plankton communities. For example, bulk metrics of phytoplankton and zooplankton abundance show that the years 2014 to 2016 had very low numbers of large diatoms but high numbers of zooplankton (Fig. 1). This could be the result of top down impacts with the abundant zooplankton consuming the diatoms, perhaps coupled with poor growing conditions for the phytoplankton cells. In 2017 large diatoms were very numerous and zooplankton were still abundant even though the heatwave had diminished the plankton had not returned to typical levels, perhaps pointing to changes further up the food chain.
To investigate the changes in the communities at the level of individual taxa, we calculated Community Temperature Indices (CTI) for phytoplankton and zooplankton. This approach utilizes abundance data for each individual taxon and the in-situ temperature from the physical data collected via instruments attached to the CPR to first calculate a Species Temperature Index for each taxon. Then, to assess how communities shift over time, the CTI is calculated for each sample and averaged for the year. The results are shown below (Fig. 2). The relationship with sea surface temperature (SST) demonstrates a shift to communities comprising organisms that live in warmer waters when SST is higher, although for phytoplankton this is only significant if 2015 is excluded (note that because the abundance of phytoplankton was very low in 2015 and 2016, this index may not be as sensitive here since many taxa were simply not present). What is noticeable, however, is that while there is a clear shift towards warmer communities, the heatwave years of 2014-2016 were not noticeably different from the preceding warm period in 2004-05.