Oceanographic Environment of Glacier Bay

Oceanography describes one of the most fundamental physical aspects of a marine ecosystem. Glacier Bay exhibits a highly complex oceanographic regime within a small area.  An understanding of many of the resource and research issues in Glacier Bay will not be possible without an understanding of the underlying oceanographic processes causing the large spatial and annual variation found within the Bay.
 

The Bay is a recently (300 years ago) deglaciated fjord located within Glacier Bay National Park in Southeast Alaska.  Glacier Bay is a fjord estuarine system that has multiple sills. These sills are often associated with contractions and are backed by very deep basins with tidewater glaciers and many streams.  Glacier Bay experiences a large amount of runoff, high sedimentation, and large tidal variations.  Melting occurs year-round, which is thought to fuel the estuarine circulation even through the winter.  This runoff, and the presence of the tidewater glaciers makes the bay extremely cold.  There are many small- and large-scale mixing and upwelling zones at sills, glacial faces, and streams. The complex topography and strong currents lead to highly variable salinity, temperature, sediment, productivity, light penetration, and current patterns within a small area.   This complexity defies simple characterization or modeling based on other areas in Southeast Alaska.  While several oceanographic studies have been conducted in Glacier Bay, these studies are contradictory and were of short duration and limited coverage, missing much of the spatial, seasonal and annual variation.  In addition, some assumptions based on past studies have been contradicted by recent results.  The constantly changing nature of the Bay may contribute to contradictions among past studies and between recent and historical results.

The primary data used in this study (see the Fjord Oceanographic Monitoring Handbook) were oceanographic surveys consisting of samples of 24 central-channel stations from the mouth of the bay to the heads of both the East and West Arm taken between 1992 and 2000 . A oceanographic instrument (CTD) capable of recording depth, temperature, salinity, light penetration, amount of sediment, and amount of phytoplankton was used to obtain measurements at one-meter intervals throughout the water column to a depth of 300m at each station.  Surveys were conducted up to seven times a year in such a manner as to encompass the primary annual variation in oceanographic patterns.

Results from the current work indicate several shifts in the dominant paradigm of oceanographic understanding for this area.  Primarily, deep-water renewal, and with it increased nutrient availability, is not limited to the winter months but can and probably does occur regularly in the spring/summer/fall at least during spring-tide series.  We believe that Glacier Bay is not a traditional silled fjord estuary nor a plain fjord estuary like most estuaries in Southeast Alaska, but a combination of a stratified deep basin estuary and a tidally mixed estuary.  This results in a complete blockage of estuarine circulation at the mouth of the bay; instead, mixing occurs by turbulent diffusion rather than buoyancy-driven entrainment as seen in a traditional estuary.  In addition, where this turbulent water meets the deep stratified basin, a tidally-mixed front is created.  This front is ideal for encouraging high primary productivity by phytoplankton.  Mixing phenomena have now been observed to be much more extensive in Glacier Bay than previously thought.  This, as with the increased deep-water renewal, almost certainly results in a more nutrient enhanced system.  The Bay is now known to exhibit phenomenally high primary productivity, which is sustained throughout the summer in sharp contrast to other interior waters of Southeast Alaska and adjoining shelf and oceanic areas.  The limits on this productivity appear to be caused not by grazing by zooplankton or nutrient limitation, but by light penetration.  Light penetration in Glacier Bay is significantly reduced by both sedimentation phenomena related to the young nature of the terrestrial environment and to the high levels of the primary productivity itself.  In addition to these major changes in understanding, we have identified several new phenomena that will need further study to understand their nature and significance.

As a result of the oceanographic monitoring program and the synthesis effort funded by the NPS there is now an extensive body of oceanographic and weather data integrated into the Glacier Bay information management system and available on CD-ROM.  The Oceanographic Analyst Extension, a Geographic Information System (GIS) tool, has been created to allow viewing, analysis, and manipulation of these complex data in 3 and 4 dimensions.