Hey everyone, Scott Gabara here to describe part of my dissertation working on creating food webs of nearshore and offshore subtidal communities across the Aleutians Islands. In the marine environment, kelps can create the understory and canopy that comprise most of the biomass of kelp forests, providing structure and food resources for their associated communities and can even deliver material to adjacent areas supporting consumers many kilometers away! This is very similar to the plants, bushes, and trees that comprise forests on land (and can deliver leaf litter to adjacent areas).
With the loss of sea otters and subsequent formation of urchin barrens across many locations of the Aleutian Island archipelago, kelp is lost and organisms will either not be able to deal with the loss of food that kelps provided, or they may change their diet to another source of food locally, or rely on food from other locations that drift in. At some locations urchin barrens were formed long ago, but most formed during the late 1990s- early 2000s, giving us communities that lost kelp long ago, many that have lost kelp for about 20 years, and some locations that have had kelp continuously. I wanted to better understand the long term impacts of kelp loss on the trophic ecology of these kelp forest communities that still have kelp or have lost kelp over different amounts of time. To do this I will create and compare food webs using stable isotopes of the dominant community members (producers and consumers) among urchin barrens, kelp forests, and offshore communities at islands across the Aleutian archipelago.
Stable isotopes of carbon and nitrogen are stored in tissues and help us to understand how energy flows through food webs. Stable isotopes act as a hidden tracer that can reveal both where energy is coming from, and the trophic level of a consumer. For example, we can sample the fingernails or hair of people, and using carbon stable isotope values, be able to tell differences of diets based more on rice versus those based more on corn because of the differences in the way rice and corn take carbon up during photosynthesis. We can also identify differences in the diet of people that are vegan, vegetarian, or those that eat more meat, because of differences in the trophic level of the foods that they eat. All of this information is stored in the nitrogen stable isotopes of the food we eat and is then left as a signature in our fingernails and hair.
Food webs help us to understand the connections among organisms and reveal the potential impacts of removing certain food resources (primary producers) that comprise the base of the food web or the impacts of removing the food web members (consumers).
Potential food web resources for kelp forest and urchin barren associated organisms are things in the water such as phytoplankton (that live in the water column) or pieces of algae that break off while algae grow or get ripped off the bottom and drift around, multiple attached algae such as the lower lying green and red algae in kelp forests, and the larger subcanopy and canopy forming brown algae.
To estimate a water column isotopic signature, water was filtered from urchin barrens, kelp forests, and offshore. Filtering water for phytoplankton or algae particulates is no easy task. Water is collected and then needs to either be pushed or pulled through a filter to concentrate the material from the water. To do this I modeled a water filtration setup after the Rohwer lab at SDSU/Smith lab at Scripps developed by Mark Hatay at SDSU. Clear PVC was cut to fit 5 Liters of water, 4 inch adjustable plumbing plugs were used to seal the ends, and tapped plastic valved fittings onto the plugs were used to create a tube that could collect water and be pressurized. A propane pressure regulator and a SCUBA tank was used to create pressure that would push the water through the clear collection tube and then through a glass fiber filter in a holder, concentrating the material from the water on the filter, to be dried and later run for stable isotope analysis.
Locations of nearshore and offshore (grey), kelp forest (green), and urchin barren (blue) water samples.
To estimate algal isotopic signatures, tissues of algae that were living in urchin barrens and kelp forests were collected. To estimate the isotopic signature of nearshore and offshore consumers, invertebrates were collected from surveys or offshore trawls done in collaboration with the Konar lab at University of Alaska Fairbanks. To estimate fish isotopic signatures, pole spears were used to catch fish that were in urchin barrens and kelp forests by divers and by using the trawl to get fish that were living in the deeper offshore areas.
Tissues were prepped and dried on the R/V Oceanus and will be run for carbon and nitrogen stable isotope analysis. The ratio of the heavier to lighter stable isotopes of carbon and nitrogen will help us trace how important local and/or drifting kelp is for organisms in urchins barrens, kelp forests, and offshore areas. By creating food webs of these different communities we may have a better understanding of why certain organisms can survive the transition from kelp forests to urchins barrens and if and how kelp loss in the nearshore affects offshore food webs.