Glacier silt hinders establishment of kelp bed communities

01/12/2016

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NR: SG-2016/NR296
Kim Powell replaces tubes on a sediment trap.Kim Powell replaces tubes on a sediment trap. Photo by Sarah Traiger. (Click image for larger version, 3.0 MB.)

Kachemak Bay, Alaska—Long-term build up of silt from glacier melt is limiting growth and establishment of kelp beds in Kachemak Bay, according to an ongoing project by University of Alaska Fairbanks researchers. 

Kelp beds form tall underwater forests that are an important habitat in Kachemak Bay near Homer. Large kelp beds serve as a nursery for fish and invertebrates and protect shorelines from coastal erosion. People living near Kachemak Bay harvest kelp, as well as chiton and crabs that live in kelp forests.

Brenda Konar, a professor at the School of Fisheries and Ocean Sciences, is working with PhD student Sarah Traiger to study the impact of glacier melt on the ability of kelp to travel, settle and grow on rocky substrate in Kachemak Bay, known as kelp recruitment and settlement.

“We really need to get a better handle on what is controlling these kelp forests, not just in Kachemak Bay but everywhere in Alaska,” Konar said.

Kachemak Bay supports a large diversity of kelp that relies on availability of light and rocky substrate. Kelp need hard rock to attach to and grow on. When silt carried by glacier melt settles on rock, kelp beds have difficulty establishing on that rock. In addition, suspended silt can block light from penetrating the bottom layers of the bay, making it hard for kelp to photosynthesize.

The source of glacier melt in Kachemak Bay is the nearby Harding Icefield. Different parts of the bay receive varying amounts of glacier meltwater based on whether they are upstream or downstream of glacier discharge points.

A study by the National Park Service found that the Harding Icefield has shrunk by 3 percent over the past 16 years. “As glaciers on the Harding Icefield continue to melt, we need to understand how communities and ecosystems are likely to change,” said Konar.

The research is funded by Alaska Sea Grant, with additional support from the UAF Center for Global Change and the Robert Byrd competition through SFOS.

Konar and Traiger sampled at three sites in inner Kachemak Bay near a glacier discharge point where there was substantial expected influence from glacier melt. They also sampled at three sites in outer Kachemak Bay far from discharge points, with little or no expected influence from glacier melt.

The researchers placed flat rocks at each of the study sites where the water depth was 10 meters at low tide. The rocks were big enough that they did not shift with water motion. Konar and Traiger measured the species that settled and grew on each rock, and recorded temperature, salinity, light, nutrient concentrations and sediment rates at each study site.

They did the field research during the summers of 2013 and 2014. Some rocks were left in the water for one summer field season, while other rocks were left in place from the beginning of the 2013 field season to the end of the 2014 field season.

“We began to see that other organisms influenced kelp establishment in addition to silt from glacier melt,” Traiger said. “Before silt built up on the rocks, grazing organisms played a larger role in deterring kelp recruitment and establishment.”

Over the course of one field season, grazing animals like sea urchins and lacuna snails had the strongest effect on what was able to settle and grow on the test rocks. But as sediment built up, the cumulative effect of the sediment on the rocks became the primary driver in keeping kelp from settling and growing on the rocks.

This trend was even clearer when rocks from different sampling locations were compared. “Over the longer period of time we’re seeing bigger differences in the communities across the bay with varying levels of influence from glacier melt,” Traiger said.

“This research will give us the baseline understanding of what we should monitor to figure out what is changing and what is likely to change in the future,” Konar said. “It will allow us to evaluate what might happen to populations downstream if glacier melt continues to increase over time.” The results will help managers and decision makers predict and prepare in advance for changes, Konar said.

The researchers are also exploring the ability of kelp spores to spread around Kachemak Bay. Certain regions of the bay have kelp spores that spread to populate other parts of the bay. “We are using genetics as a tool to track the presence or absence of spores in the water column, from which we can gain an understanding of their dispersal,” Traiger said. Traiger and Konar are the first researchers to use these techniques to study source populations of kelp for Kachemak Bay.

If kelp can’t populate a part of the bay, Traiger and Konar may be able to use genetic tools to answer why the kelp spores couldn’t make it from their initial location, or whether they were able to travel but unable to settle because of environmental variables such as silt and grazers.

Designated kelp harvesting areas in Kachemak Bay currently are not based on kelp biology. The results of this work could help identify where the important sources of kelp spores are, which would help decision makers determine where kelp harvesting should be prohibited.

Story by Lauren Frisch, lcfrisch@alaska.edu

Alaska Sea Grant is a statewide marine research, education, and outreach program, and is a partnership between the University of Alaska Fairbanks and the National Oceanic and Atmospheric Administration. Alaska Sea Grant Marine Advisory agents provide assistance that helps Alaskans wisely use, conserve, and enjoy marine and coastal resources.