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
Pictorial Representation
Since the inception of our LTER site we have striven to develop a conceptual model that encapsulates the NGA biome and primary scientific foci. Our goals in crafting this model are:
- To readily communicate these elements of our program to a broad scientific audience, with an eye to ecosystem inter-comparison efforts; and
- To provide a tool for internal integration of NGA elements across our various areas of investigation and effort.
Our current model integrates a box model diagram, a pictorial representation of our biome and its seasonal variability, and a visual of variability at the decadal scale.
Flexibility
We are in the process of developing two more-targeted versions of the conceptual model for two key NGA taxonomic groups: zooplankton and seabirds. These targeted versions will integrate specific examples of important ecological and life history strategies, including seasonal cycles, into the more general conceptual model of the ecosystem. The modular flexibility of our conceptual model design will allow us to communicate specific program elements to targeted audiences within an underlying, common ecosystem framework.
The box model readily maps onto more specific project components, including our current river plume process focus, suggesting we are on the right track.
Theoretical Basis
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 Property | Significance | Assessment |
|---|---|---|
| Pronounced spring bloom | Largest annual phytoplankton biomass & production signal | Satellite ocean color; in situ chlorophyll; primary production |
| Regions of sustained high summer production | Predictable 'islands' of biomass during low production season | Satellite ocean color; in situ chlorophyll; primary production |
| Stable base of energy-rich zooplankton grazers | Buffer and stabilize interannual variability in primary production | Abundance and taxonomic data; production, lipid content |
| Substantial inking flux of organic matter | Fuels benthic communities | Sediment traps; LISST-DEEP & UVP5 optical particle sensors |
| Efficient transfer of primary production to higher trophic levels | Supports high production of fish, birds, and mammels | Biomass ratios among trophic levels; feeding experiments |
Current Understanding of NGA 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.