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Ecological Framework

Intense environmental variability coupled with a rich and diverse ecosystem characterize the NGA LTER site. Heat, winds, freshwater, and light constantly change because of storms, seasons, and even glacial advances and retreats. Simultaneously, the high biological productivity during the spring bloom and at hot spots during the summer sustain higher trophic levels. Our central premise is that species and communities in the NGA behave, function, and are composed in ways that allow the ecosystem to recover from disturbances. In other words, intense environmental variability leads to high resilience in the northern coastal Gulf of Alaska. This is the conceptual framework for the NGA LTER.

Conceptual Model

Gunderson resilience figure
Ball-and-cup model of system stability. Valleys represent the system components, and arrow represent disturbances. Ecological resilience is described as the width of the valleys in the stability landscapes. credit: Gunderson, 2000.

Our research program springs from a conceptual model based on this ecosystem resilience. In this model, resilience is defined as “the amount of change or disruption that is required to transform a system from being maintained by one set of mutually reinforcing processes and structures to a different set of processes and structures” (Levin and Lubchenco, 2008). Resilience contrasts with stability – the propensity to resist change (Holling, 1973; Gunderson, 2000). Notably, because it doesn’t focus on individual elements, it depends on the concept of emergent properties – the characteristics arising from the NGA ecosystem as a whole.

The past decade and a half of study have defined the critical emergent properties of the NGA, and suggest ways in which they can be assessed.

Emergent Properties of the NGA

Emergent PropertySignificanceAssessment
Pronounced spring bloomLargest annual phytoplankton biomass & production signalSatellite ocean color; in situ chlorophyll; primary production
Regions of sustained high summer productionPredictable 'islands' of biomass during low production seasonSatellite ocean color; in situ chlorophyll; primary production
Stable base of energy-rich zooplankton grazersBuffer and stabilize interannual variability in primary productionAbundance and taxonomic data; production, lipid content
Substantial inking flux of organic matterFuels benthic communitiesSediment traps; LISST-DEEP & UVP5 optical particle sensors
Efficient transfer of primary production to higher trophic levelsSupports high production of fish, birds, and mammelsBiomass ratios among trophic levels; feeding experiments

Current Understanding of NGA Resilience

variability and resilience

From the description of the NGA biome, it is clear that the NGA has many qualities that might predictibly decrease resilience, such as:

  • Relatively low biodiversity
  • Short food chains, and
  • Top-down control of production

However, responses to past disturbances such as a recent warm-water event (the Blob) suggest that the NGA is instead a particularly resilient ecosystem. There are qualities at several scales that can increase resilience, including:

  • Intense environmental variability;
  • A complex mosaic of conditions, resources, ecosystem properties; and
  • Connections with different biogeographic provinces, including
    • Fjords,
    • The Bering Sea (downstream), and
    • The California Current (upstream)

Adaptations at the Species Level

At the species level, “bet hedging” and nutritional plasticity increase resilience. Specifically, the life histories of Neocalanus copepods are flexible, allowing them to hedge their bets. For example, they spawn at depth during the winter, and their arrival times in the surface are staggered; this might compensate for variability in the timing of the spring bloom. Alternatively, many zooplankton in the NGA feed on a wide range of particle types. Some even retain chloroplasts from the phytoplankton they eat, allowing them to generate their own food.

Adaptations at the Community Level

There are several different species of Neocalanus copepods in NGA that have slightly different life histories. Therefore, when environmental conditions vary, different species succeed. However, their functional redundancy ensures that a relatively high biomass of large copepods remains in spring and early summer from year to year.