The NGA LTER site will focus on a mechanistic understanding of processes that underlie environmental variability, and the role of the latter in promoting high productivity and resilience. Building on prior knowledge, the investigators will test three hypotheses centered on ecosystem emergent properties:
1. Changes in the hydrologic cycle affect spring bloom production through changes in cloud cover, the stratification/mixing balance, macro- and micronutrient supplies, and transport pathways.
2. Hot-spots of high summer primary and secondary production result from interactions between the fresher Alaska Coastal Current and more saline offshore waters as promoted by shelf morphology and regional winds; hot spot timing and magnitude will be influenced by changes in the hydrologic cycle.
3. Nutritional and life history patterns of NGA consumers minimize trophic mismatch, buffering spatial and temporal variability in lower trophic level production and leading to resilience in the face of long-term climate change.
The investigators will address these hypotheses with an integrated research program that includes:
a) seasonal time series studies addressing short- and long-term environmental and ecosystem variability through a spring-to-fall field cruise- and mooring-based observational program, building upon and enhancing the Seward Line times series, and leveraging existing collaborations to obtain higher trophic level data;
b) process studies that focus on hypothesized mechanisms leading to variability and enhancement of NGA production in time and space;
c) modeling studies that incorporate physical and biogeochemical observations, provide a framework for testing hypotheses, and predict ecosystem responses to projected environmental changes;
d) a data management component that provides a public platform for data visualization and synthesis by LTER colleagues, educators & students, and resource managers.Read Less
The central feature of the subarctic marine biome is strong seasonality of environmental drivers (heat, winds, freshwater input, and light) and salinity-dominated control of the water column stratification. While seasonality lends a quasi-predictable nature to the subarctic production cycle, a hallmark of this environment is marked variability in the timing and frequency of production events, as well as in the spatial expression of these events. Environmental variability over longer time scales also characterizes the subarctic biome, from multi-decadal fluctuations and trends to latitudinal excursions of the subarctic biome in conjunction with glacial retreats and advances. Thus the primary agents of disturbance in the NGA are weather events and progressive climate change.
The deep (200-300 m) continental shelf of the NGA is bounded by steep, snow- and ice-clad coastal mountains inshore and a trench offshore, cross-cut by deep canyons, and linked to numerous complexes of fjords and sounds. Low pressure systems move through the region year-round, but are stronger and more frequent in winter. Associated winds drive the circulation of the Alaska Gyre, while winds plus runoff from the copious precipitation drive the coastally-confined Alaska Coastal Current. These current systems define a series of quasi-persistent cross-shelf habitats for plankton.
Other mesoscale features important for planktonic production and distribution include river plumes, mesoscale eddies, and mixing zones (canyons, banks) associated with the large semi-diurnal tides. Regulation of primary production in the NGA is complex. Important roles are played by macronutrients (nitrogen, silicic acid), light, and grazing. Production in the outer portion of the study region can be strongly limited by iron, a micronutrient whose availability dictates the ecosystem structure of the offshore (oceanic) Gulf of Alaska.
Periods and locations of high primary production are generally associated with communities of chain diatoms, while nanoflagellates and picocyanobacteria are prevalent in most of the NGA in summer and fall. The fate of NGA primary production is poorly constrained, especially with respect to detrital pathways. Prior research indicates that grazing removal of primary production is primarily by protists (‘microzooplankton’), while larger zooplankton are key in transferring energy and biomass from the microbial to the vertebrate communities.
Large, lipid-rich copepods of the genus Neocalanus dominate the spring zooplankton biomass, and are important prey for higher trophic levels. Their long life cycle, with an over-wintering resting period coupled to early spring reproduction, couples production across years and may buffer the system against environmental variability.
While numerous species of salmon are present in the NGA, fish communities are dominated by piscivorous demersal species, notably arrowtooth flounder, walleye pollock, Pacific cod, Pacific Ocean perch, and Pacific halibut. A range of other flatfish, rockfish, and shellfish (shrimp, crab) contribute to harvested communities, while lipid-rich forage fish (e.g., Pacific herring, capelin, sandlance, eulachon) are important trophic intermediaries for piscivorous fishes and seabirds. The predominance of demersal and benthic species at higher trophic levels suggests high rates of production exported to the benthos, at least seasonally.Read Less
- 1970-present: hydrographic (temperature, salinity) sampling of station GAK1 (nearshore end of Seward Line transect)
- 1997-present: oceanographic sampling of Seward Line transect and Prince William Sound
- 1997-2004: intensive observation and process studies supported by U.S. GLOBEC program
- 2005-present: observations supported by a consortium of funders
- 2011, 2013: intensive observation studies supported by North Pacific Research Board’s Integrated Ecosystem Research Program
The NGA LTER builds on 20 years of multidisciplinary oceanographic observations along the Seward Line and within the Prince William Sound fjord complex, and a 45-year hydrographic time series at station GAK1. Hydrographic observations began in the mouth of Resurrection Bay at Station GAK1 in 1970, and have been conducted ~monthly since the early 1970s.
The Seward Line sampling transect stretches 150 nautical miles from GAK1 at the mouth of Resurrection Bay into oceanic waters well past the continental shelf break. Sampling of the Line began as part of the intensive Northeast Pacific GLOBEC program (NSF- and NOAA-supported) in October 1997, with long-term observation cruises (~6x per year, 1997-2004), mooring deployments, and process studies (2-3x per year, 2001 and 2003).
These studies established seasonal cycles and rate processes for the region. From 2005-2010, sampling frequency was reduced to May and September only, primarily along the core Seward Line stations, and with support by the North Pacific Research Board (NPRB). By 2012, a consortium of NPRB, the Alaska Ocean Observing System (AOOS), and the Exxon Valdez Oil Spill Trustee Council contributed to a sampling program with increased effort inside Prince William Sound and consistent addition of seabird/mammal observations.
During 2011 and 2013, a more intensive observational effort in the NGA was conducted through an NPRB-funded Integrated Ecosystem Research Project. This program allowed expanded along-shelf coverage during spring, summer and fall, and explicitly linked oceanographic conditions and plankton communities with early life history stages of commercially important demersal fish species.
Core historical time-series from the above sampling efforts include measurements of temperature and salinity, macronutrients, chlorophyll, and zooplankton. Other historical data sets relevant to the NGA include NOAA’s ichthyoplankton surveys (annual from 1977-2011; biennial thereafter) and groundfish surveys (triennial from 1980-1999; biennial thereafter). In 2017, the Seward Line program joined the LTER network, expanding the sampling domain, adding an early summer cruise, adding a suite of additional oceanographic measurements, and allowing targeted study of features believed to contribute to the high productivity of the northern gulf.Read Less