The Northern Gulf of Alaska (NGA) is tucked into a corner of the North Pacific. It is a subarctic marine biome that occupies the deep (200-300 m) continental shelf there. This shelf transitions between two distinct physical environments. Inshore, fjords and sounds link the NGA to steep, snow- and ice-clad mountains. Offshore and between sub-marine banks, deep canyons cross-cut the shelf, leading to a deep oceanic trench.
Atmospheric forcing in the NGA is dominated by low pressure systems which sweep the area year-round; however in winter, storms are stronger and more frequent. Winds associated with these systems drive the large-scale, oceanic circulation of the Alaska Gyre – including the Alaskan Stream offshore. Inshore, abundant precipitation in the mountains combine with downwelling winds to drive the coastally-confined Alaska Coastal Current (ACC). These two current systems create a series of quasi-persistent cross-shelf habitats for plankton.
Environmental drivers in the NGA include strongly seasonal conditions like heat, winds, freshwater input, and light. Previous studies such as GLOBEC and GOAIERP have already defined the quasi-predictable effects of these environmental drivers on the biome. However, the primary causes of ecosystem disturbance, which is of key interest to the LTER program, occur on other temporal scales.
At shorter time scales (hours to days), weather events cause the timing and frequency of biological production events to dramatically vary in ways that might depend on location. Also at short time scales, distribution of plankton and production are impacted by other features like:
- River plumes,
- Mesoscale eddies, and
- Mixing zones associated with large, semi-diurnal tides over canyons and banks.
Phenomena with longer time scales are also important. These include multi-decadal fluctuations and trends such as the Pacific Decadal Oscillation (PDO), and millennial excursions of environmental conditions as glaciers retreat and advance.
The cross-shelf gradients of available nutrients mirror those of the NGA’s physical environment because there are distinct conditions inshore and offshore. Offshore, waters typically have high levels of macronutrients and low chlorophyll (HNLC); biological production in these areas is limited by micronutrients and trace metals such as iron.
Inshore waters are characterized by high iron levels. Iron is input to this area via sediments delivered from the land by freshwater like rivers and glacial meltwater. Macronutrients (nitrogen, silicic acid) on the shelf episodically increase in response to environmental drivers. For instance, nutrient-rich water is mixed upward by storms in the winter, and is advected onto the shelf by eddies or downwelling winds. As the summer progresses, nutrient levels decrease as the water re-stratifies, and nutrients are consumed without being replenished.
A bloom of primary production dominates the ecosystem in the spring, when higher levels of nutrients and light support higher populations of phytoplankton. Generally, within periods and locations of high primary production we measure communities of beefy chain diatoms. In contrast, background levels of smaller species like nanoflagellates and picocyanobacteria prevail in most of the NGA in summer and fall. The factors that regulate primary production vary partly on cross-shelf location. Inshore, macronutrients, light, and grazing all play roles, whereas offshore, iron limits production.
Several pathways enable transfer from primary production to higher trophic levels. Microzooplankton are key consumers in the NGA. They consume about half of the diatom production and nearly all the small phytoplankton production, and their populations closely mirror those of primary producers. Lipid-rich Neocalanus copepods and other larger zooplankton transfer energy and biomass from the microbial to the vertebrate communities. Zooplankton play this key role because they:
- Dominate the spring zooplankton biomass,
- Are important prey for higher trophic levels, and
- Have a long life cycle with an over-wintering resting period followed by reproduction in the early spring.
When zooplankton production spans multiple years, this may allow species to take advantage of conditions when they are favorable and wait out unfavorable conditions. In this way, they are protected from environmental variability, which adds resilience to the entire system.
Other Higher Trophic Levels
Piscivorous demersal species (arrowtooth flounder, walleye pollock, Pacific cod, Pacific Ocean perch, and Pacific halibut) dominate fish communities. Numerous species of salmon and a range of other flatfish, rockfish, and shellfish (shrimp, crab) contribute to harvested communities. Lipid-rich forage fish (e.g., Pacific herring, capelin, sandlance, eulachon) are important trophic intermediaries for piscivorous fishes and seabirds. Demersal and benthic species predominate at higher trophic levels, which suggests production is exported to the benthos at high rates, at least seasonally, via detrital pathways.