Terrestrial Influence on Glacial-Marine Food Webs

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Where glaciers meet the sea in the Gulf of Alaska (GOA), they create unique and productive marine habitats. Ringed by the continent’s tallest coastal mountains, 20% of the GOA coastal watershed is covered by glacial ice and the annual freshwater discharge into the GOA from glacial melt is comparable to that of the Mississippi river.

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Kittlitz's Murrelet flying above the water in Cook Inlet, Alaska

Kittlittz's Murrelet flying above the water in Cook Inlet, Alaska

(Credit: Sarah Schoen, USGS, Alaska Science Center. Public domain.)

Freshwater influx drives the Alaska Coastal Current which in turn sustains commercial and subsistence fisheries as well as millions of coastal marine birds and mammals. Nearly half of this freshwater is derived from glacial melt-water, but we know little about how this glacial runoff contributes to marine production in glacial-marine ecosystems. This question is timely given the widespread and rapid retreat of glaciers in Alaska owing to global warming. Water and organic nutrients derived from terrestrial sources have distinct isotopic signatures compared to those of marine origin. We hypothesize that organic nutrients of terrestrial origin significantly enhance secondary and tertiary productivity in glacial-marine pelagic food webs. This study traces the flow of nutrients and organic matter through marine food webs using stable and radio isotopes. To better understand the scale at which glacial runoff influences food webs in coastal waters, we also examine the extent of freshwater infiltration and mixing into the marine system by mapping the water isoscapes in bays and fjords.

Project overview: To demonstrate linkages between terrestrial and marine ecosystems, we are using stable and radio isotopes to estimate the relative contribution of glacially-derived organic matter and freshwater to marine food webs. During summers of 2012 and 2013, we sampled dissolved and particulate organic matter, zooplankton, forage fish and seabirds near tidewater glaciers in Prince William Sound, Alaska. We also collected water samples to measure the extent of mixing and infiltration of glacial melt water in the marine system. We anticipate that δ2H, δ13C and radiocarbon signatures of lower trophic marine organisms will permit us to trace glacier-derived freshwater and organic matter in marine food webs owing to the markedly different isotope ratios between glacio-terrestrial versus marine hydrogen and carbon sources. We also expect δ2H and δ18O isoscape mapping to clearly show the pattern of freshwater mixing into the marine environments.

Project objectives: Although most agree that the freshwater contribution from glacier ice fields is important in maintaining the density gradient that drives the Alaska Coastal Current, the importance of this freshwater source of highly bioavailable organic matter to marine ecosystem function is not well studied. Therefore, the overarching goal of this work is to measure the contribution of terrestrial-derived organic carbon and deuterium on nearshore heterotrophic production, and to trace the distinct isotopic signatures associated with glacier-derived organic matter through the marine food web.

  1. Estimate the importance of freshwater-derived organic matter and organic nutrients to nearshore marine production using 13C and 2H stable isotopes
  2. Examine whether glacier-derived ancient organic matter subsidizes the nutritional requirements of marine biota using radiocarbon Δ14C
  3. Model the spatial pattern of freshwater infiltration and mixing in glacial fjords in the Northern Gulf of Alaska using 2H and 18O stable isotopes

Methods: To quantify the contribution of terrestrial-derived organic matter and nutrients to glacial-marine ecosystems in the Gulf of Alaska, we sample food web components from organic material (OM) and on up the food chain to marine birds. Species were selected to allow comparison of a range of pelagic feeding modes (local and wide-ranging planktivores and piscivores). Samples from focal group or species, and to sample both glacier-proximal and glacier-distant marine sites in each study area (recognizing that it is difficult, if not impossible, to escape some degree of glacial influence at any coastal area of the northern Gulf of Alaska). All POM and biota samples will be analyzed for stable isotopes δ13C and δ2H. To further examine ancient carbon subsidies to nearshore pelagic food webs, we will analyze a subsample of OM and biota samples (50 samples total per year) for radioisotope Δ14C. We will also map isotopic variability within the glacier fjords using δ2H and δ18O signatures. Isoscapes will allow us to model the pattern of freshwater and glacial runoff infiltration and mixing in fjords.

Water and Organic Matter: Water samples are collected with a SeaBird Electronics CTD and Ecosampler® at depths of 10 meters and 2 meters. The water samples are analyzed for stable isotopes, radio isotopes, nutrients, chlorophyll, and dissolved inorganic carbon.

Zooplankton: Zooplankton samples are obtained by lowering a paired 150 micron mesh zooplankton net down to 50 meters or within 5 meters of the seafloor. From the zooplankton samples we calculate species composition and abundance, and copepod samples are analyzed for stable and radio isotopes.

Many types of small fish laid out on the table for counting

Catch of forage fish from a trawl in Prince William Sound, Alaska. Trawling was part of detecting long-term changes in forage fish populations.

(Credit: Mayumi Arimitsu, USGS, Alaska Science Center. Public domain.)

Forage fish: Fish and euphausiids are collected using a modified herring trawl. The entire catch is identified to species and enumerated, larger catches will be subsampled by volume. Our goal is to obtain 15 samples of each target species for stable isotopes.

Seabirds: A variety of seabirds are captured using noose poles, noose mats, nets, and night lighting. We collect a small amount of blood from each bird to analyze for stable and radio isotopes.