What factors determine the health, growth and productivity of giant kelp? There are several, but according to researchers at UC Santa Barbara and the Woods Hole Oceanographic Institution (WHOI), what you see depends on the scale at which you’re observing.
“Depending on your spatial scale of observation – whether you’re looking at kelp forests regionally or really focusing on a specific local area – the patterns that show up at those scales can indicate different factors,” Tom said. Bell, a project scientist at UC Santa Barbara’s Earth Research Institute and assistant scientist at WHOI, and senior author of an article published in the Proceedings of the National Academy of Sciences. Being able to observe the different drivers of giant kelp productivity and health at different scales, he said, can provide a more nuanced and current understanding of how kelp forests respond to changing climatic conditions. .
A founding species
Giant Kelp (Macrocystis pyriferous) are the largest seaweeds in the world. Spanning over 100 feet in height, they can be found in dense areas along the California coast. They are home to a wide range of marine animals that depend on them for food, shelter and raising their young. People also rely on kelp forests for recreation and to support and attract fish and other seafood.
“Giant kelp is a fundamental species and plays an outsized role in the dynamics of kelp forest ecosystems,” said David Siegel, professor of oceanography at UCSB and co-author of the paper. As a result, he added, resource agencies and conservation groups are interested in monitoring and managing these ephemeral systems using new remote sensing tools developed at UC Santa Barbara.
But what controls changes in the canopy cover of a kelp forest? According to the researchers, discerning this requires assessing the kelp’s external interactions with the environment, as well as their internal biological processes — specifically the senescence, or age-related physical deterioration of the kelps over time. Making adequate assessments of intrinsic and extrinsic factors has been a challenge, they say, due to “the inability to track both abundance and plant health at appropriate scales”. Field measurements tend to occur in discrete areas on small scales, and the lack of relevant measurements on large continuous scales makes it difficult to disentangle how environment and age demographics influence kelp populations. .
A bigger picture
Enter remote sensing technology. Using data collected by satellite and aircraft, Bell and Siegel focused on a 4,000 square kilometer study area in the Santa Barbara Channel, a section of the Coast Long-Term Ecological Research Site. of Santa Barbara from UCSB. The researchers used three decades of satellite imagery to assess canopy biomass and age dynamics. They also used repeat imaging spectrometer data collected by plane from 2013 to 2015 to estimate the chlorophyll to carbon ratio – an established indicator of physiological status. To these large spatial measurements, they added longitudinal and repeated field observations of kelp health and productivity to assess the roles of kelp frond age and environmental conditions on kelp population dynamics.
The researchers found that the effects of different drivers were more pronounced at different scales.
“At the regional scale for areas greater than one kilometer, seawater nutrients were related to the dynamics of kelp physiological state,” Bell said. “However, at local scales of less than one kilometer, internal senescence processes related to age demography of the kelp canopy were associated with patterns of biomass loss in individual kelp forests, despite conditions nutritional uniforms.” Thus, the growth of kelp fronds and the initiation of new ones are influenced by the regional concentration of nutrients in the water. But over time and on a small scale, internal biological processes take over and older fronds begin to deteriorate, regardless of nutrient availability.
“We didn’t know that senescence would lead to such interesting spatial patterns in the canopy at the local scale,” Bell said. “It was previously thought that when seawater nutrients decline in summer, the entire canopy would respond in the same way. This research shows that different parts of the forest age and are lost sooner than other parts, and it is important to understand why seawater nutrient dynamics do not always correspond to kelp dynamics.Although the intrinsic small-scale phenomenon of senescence is the primary driver of kelp canopy frond dynamics , he added, this does not exclude the influence of larger external factors, such as large wave events, which could wipe out the entire canopy.
“There is an interaction between extrinsic factors controlling the population dynamics of Santa Barbara Channel kelp forests at large scales, while intrinsic factors regulate spatial patterns at smaller scales,” Siegel said. “This understanding will lead to better models of how our kelp forests will change, given climate change.”
Meanwhile, the use of new and advanced sensing technologies such as hyperspectral imaging will give ecologists “another tool in their toolbox to understand how systems change, whether those systems are lowland tropical forests or giant kelp,” Bell said. “This is going to be more important than ever as the environment becomes less predictable.” The paper, according to the researchers, provides proof-of-concept information for NASA’s upcoming Surface Biology and Geology satellite mission, which plans to use hyperspectral imagery to observe Earth processes on land and at sea, as well as the effects of human activity on the environment.
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