Chasing photons in kelp forests

Adventures in Kelp

On August 4, 2008 the Bio-Optical Research Group (BORG) headed west to California. The reason, to chase photons in the kelp forests off the Santa Barbara coast. NASA has been funding us to continue Dr. Zimmerman's studies of kelp forest dynamics using remote sensing. Our ultimate aim is to be able to monitor kelp forest abundance and productivity, and their response to environmental factors such as wave stress, sediment plumes and nutrient availability, using ocean color remote sensing based on  aircraft and satellites.

During our trip we would be working with Dr. Dan Reed at the Santa Barbara Coastal Long Term Ecological Research (LTER) group which is housed at the University of California, Santa Barbara. Working with this group allowed us access to long term measurements of biomass, distribution and productivity within the kelp forests which we could compare to our absorption, reflectance and irradiance measurements.

In addition to our NASA funded research, Meredith McPherson an undergraduate student who has been working with us for 12 months was awarded a research grant from the honors council to investigate the effect of the kelp canopy on the light environment experienced by the understory algae.

Dick, Victoria and David were already certified divers with numerous research dives under their weight belts, our undergraduate student Meredith (picture below) was a newly certified research diver in training. In addition Dick was our only member who had previous experience diving in kelp. In order to acclimate ourselves to this new environment without the added stress of carrying our sensitive instruments, we spent Sunday carrying out some practice dives at Anacapa island. This enabled us to figure out our weight requirements which had increased due to the added thickness of 14mm wetsuits needed to combat the cold. At Anacapa we found urchin barrens and giant kelp forests teaming with garibaldi (large orange fish), decorator crabs, spider crabs, nudibrachs, strawberry anemones, keyhole limpets and the odd sea lion. With one kelp dive under our belts we were ready to hit the LTER sites first thing Monday morning. 

Meredith getting ready to dive.

During our field campaign we visited four sites which are routinely surveyed by the LTER group (see image below found on Santa Barbara Coastal LTER website, www.sbc.iternet.edu). These in the order of our sampling were (I)Carpinteria, (C) Arroyo Quemado, (D) Naples Reef and  (H) Mohawk reef.  Our ride for the week was the 22 ft Kelp Fish (see below) operated by the University of California, Santa Barbara. Every morning we loaded it up with all of our equipment and headed off to our launch location.

Depending on our research site for the day we launched from Santa Barbara harbor, relatively easy, or the more difficult Goleta Pier. Launching from the pier involved swinging the kelp fish out over the water and then lowering her down 40ft to the water (see below). We then had to climb down the ladder from the pier and make our death defying leap to the kelp fish (also see below).

Research sites	The Kelp fish on land
Kelp fish being launched from Goleta pier	David jumping from pier to boat

 

Once we arrived at our study site, the first instrument in the water was the active optics package, this is a cage with several instruments attached including an ac9 which measures absorption (a) and attenuation (c) at 9 wavelengths, a fluorometer, a VSF that measures the volume scattering function and lastly a CTD to give us the depth, salinity and temperature of the water. This cage was lowered through the water column to within 2 meters of the sea bed, then brought back to the surface and tied off so it hung just below the surface of the water for the rest of our time on station.

Meredith and David lowering the optics cage.	The kelp monster (a.k.a. optics cage)	The optics cage submerged (photo by Clint Nelson)

 

The second instrument in the water was the Satlantic Hyperpro profiler, this enables us to measure both upwelling and downwelling light within the water column. David deployed the instrument and controlled the descent cable while Meredith or Victoria ran the computer to log and save the data from each repeat profile. The process was repeated both in open water outside the kelp forest and within the kelp canopy itself to determine how the floating kelp blades affect the underwater light environment. When deploying in the kelp a third person (usually Dick) was required to float at the surface on the canopy and control the descent of the instrument.

 

Dick deploying the hyperpro within the kelp canopy, and Victoria controlling data collection from the laptop.

Once our surface measurements were completed it was time to don our dive gear and carry our subsurface instruments down the anchor line to the seabed. Dick and Meredith were in charge of the DOBBS and camera, while David and Victoria deployed the RUBLE and collected water and kelp samples.

The DOBBS or Diver Operated Benthic Bio-Optical Spectrometer (Picture XX) is used to measure the downwelling irradiance above the seabed and the upwelling irradiance reflected upwards from the seabed, this enables us to measure the reflectance of the seabed or benthic macroalgae, in other words the fraction of the downwelling light that is reflected by our target of interest. The DOBBS was in Meredith's charge, once submerged a target was identified and the DOBBS placed over the object whether sand, rock or macroalgae. The instrument was then initialized and Meredith watched over it until the measurement was complete which can take several minutes depending on the intensity of the light field. The camera which is attached to a frame was placed over the area that was just measured and a picture taken, this enables us to calculate the percentage coverage of macroalgae, sand or rock.  

Meredith diving with the DOBBS. Picture by Clint Nelson	The RUBLE among the kelp. Picture by Clint Nelson

 

While Dick and Meredith were measuring reflectance of the seabed Victoria and David were using the RUBLE or Robust Underwater Benthic Light Estimator to measure the water column attenuation and collecting both water and kelp samples. The RUBLE is designed with four spectrometer sensors, two irradiance sensors placed at a vertical interval of 1 meter apart from each other allow us to measure the attenuation, or loss through absorption and scattering of light over that 1 m interval. Two gershun sensors measured upwelling and downwelling scalar irradiance and together they allow us to calculate the total scalar irradiance. While the RUBLE was cycling through the measurements, water samples were collected from the same depth and area and kelp blade samples were collected from the bottom, middle and top of the plants.

Once we had exhausted ourselves in the kelp all day, it was back to lab to process samples and recharge instruments. Meredith, Victoria and Dick filtered the water samples through membranes to collect both

 

Meredith filtering water samples and measuring absorption properties

particulate and dissolved fractions. The particulate material was used for several analyses; (1) a sample was frozen in liquid nitrogen for preservation, and will be sent off to Horn Point Labs in Maryland for pigment analysis, (2) a second sample was placed in the spectrophotometer to allow us to measure the absorption properties of the material and (3) a sample was filtered through a special membrane which had been weighed in our lab before we left for fieldwork, this sample will be dried back in our lab and reweighed, the difference between the pre-sample and post-sample weight provides the weight of non-organic material in our samples. The filtrate (water that makes it through the filter) was also collected and the absorption measured in the spectrophotometer, this gives us the absorption of Chromophoric Dissolved Organic Material (CDOM), which can be a large contributor to total water column absorption. The kelp samples were placed into tanks of flowing seawater to allow us to keep them healthy for a few days until we had time to

 

Meredith and Dick picking out kelp sample from the tanks, cutting sample disks and extracting pigments.

process them. Discs were cut out of the kelp blades and weighed, reflectance and absorption were measured and pigments extracted for quantification. David meanwhile spent every evening downloading data from the instruments, checking to make sure that the data was collected properly and charging instruments for the next day.

David checking out the day's data.

At the end of a week we had all earned our kelp diving certificates, although we were sad to say goodbye to Santa Barbara for now we headed back to ODU will lots of data to keep us busy through the winter until we once again head back to the kelp forests.

Victoria

For more information on our current research projects please visit http://sci.odu.edu/oceanography/directory/faculty/zimmerman/researchpage/index.shtml