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?
• Changes in plankton communities coincided with warmer surface water above the Gulf of Alaska continental shelf in 2014 and 2015. Spring 2015 temperatures, in particular, were almost 4 1/2°F (2°C) higher than previous years.
• Changes in plankton communities related to warmer surface water are predictive of corresponding changes in the health and productivity of animals at higher levels in the food web.
The plankton communities changed in two significant ways in response to higher water temperatures. More rod-shaped diatoms (a type of phytoplankton) and small copepods (a type of zooplankton) were present in the spring compared to previous years when more centric (round) phytoplankton and smaller copepods were more abundant. In nearly all of the zooplankton communities sampled (more than 90%), small copepods were more numerous during spring, 2015, than in all other years of CPR sampling.
The higher abundance of rod-shaped phytoplankton may indicate that this long, thin growth form with its high surface-to-volume ratio was favored under conditions of low nutrient availability in ocean surface waters. The high numbers of small copepods are an indication that the warm winter and spring water temperatures could have triggered reproduction earlier in the season than usual and allowed for multiple generations, thus high productivity, throughout the season.
These changes in the plankton community may have affected the growth and survival of animals at higher levels in the food web in a variety of ways. Rod-shaped diatoms could have had a different nutritional value than the centric-shaped phytoplankton. In addition, even though small copepods were numerous, they would be available to predators for a shorter amount of time compared to larger copepods that store energy-rich lipids necessary for overwintering.
Figure 1. 2014 AND 2015 SAW A SHIFT IN PHYTOPLANKTON ABUNDANCE FROM ROUND OR CENTRIC-SHAPED DIATOMS (BOTTOM PHOTOGRAPH) TO ROD-SHAPED PHYTOPLANKTON THAT MAY BE LESS NUTRITIOUS FOR PREDATORS.
Figure 2. IN RESPONSE TO WARM WATER TEMPERATURES, SMALL COPEPODS THAT LACK ENERGY-RICH LIPIDS DOMINATED THE ZOOPLANKTON COMMUNITY IN SPRING 2015, WHICH MAY HAVE CONSEQUENCES FOR HIGHER TROPHIC LEVEL SPECIES. THE ADJACENT PHOTGRAPH SHOWS SMALL PSEUDOCALANUS SPP ALONGSIDE THE LARGER CALANUS MARSHALLAE.