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Ecology

On the case: Scientists use many sources to find the culprit in kelp disappearance

Carnell, P. E., & Keough, M. J. (2019). Reconstructing Historical Marine Populations Reveals Major Decline of a Kelp Forest Ecosystem in Australia. Estuaries and Coasts, 1-14.

The extensive decline of the Great Barrier Reef has received a lot of press attention in recent years, yet reefs aren’t the only down-under ecosystem struggling. In a recent paper in Estuaries and Coasts, researchers Carnell and Keough from the University of Melbourne and Deakin University have reported an equally alarming decline in kelp forests in Port Phillip Bay, Australia.

Figure 1: Ecklonia radiata, or common kelp, provides shelter and habitat for many marine species. Photo by John Turnbull.

Underwater Forests

Kelp, which is a large brown algae, forms an extensive, forest-like habitat in the coastal area bordering the shore and open ocean, supporting a wide variety of species, including the sea urchin, which feasts on the kelp and shapes the underwater forest. With kelp on the decline and sea urchins seemingly on the rise in Port Phillip, understanding the complicated relationship between sea urchins and kelp requires long-term records. Are sea urchin populations a function of kelp availability, or are kelp forests a function of the number of sea urchins around? Carnell and Keough set out to determine if the health and density of kelp forests were shaped by sea urchins alone, or if other forces were at play.

Challenges

Studying long-term changes in an ecosystem can be hard. Unless we are lucky enough to have a long-term record, we can only compare what is going on now to what was going on when we first started researching, maybe only a few years ago. Often, we don’t know if a change is sudden or unusual, or if we are observing regular fluctuations that take place over a long time.

Figure 2: Heliocidaris erythrogramma, the sea urchin that is commonly found in Point Phillip Bay with kelp. Photo by John Turnbull.

For kelp forests, a lack of long-term monitoring data makes answering questions about their ecology especially difficult. Sea urchins, as well as the kelp fronds they prey on, go through natural cycles of boom and bust. As the urchin horde moves in, they eat so much of the forest that they kill large patches off and may starve as a result. As the urchins move on or die off, the kelp returns, and the cycle begins again, making kelp forests tricky to study.

A Novel Approach

To track the changes in kelp-sea urchin cycles, Carnell and Keough gathered information on kelp (Ecklonia radiata) and urchin (Heliocidaris erythrogramma) populations from a number of historical sources dating back to 1931, including past research theses, technical reports, their own field surveys, and aerial photography. They then created a fairly coherent timeline of the number of sea urchins across three locations in Port Phillip Bay: Beaumaris, Point Cooke, and Williamstown.

The researchers also aimed to factor in environmental conditions, but the records on the sea water quality only went back to 2002. Carnell and Keough had to estimate the missing information from other historical climate data. They could estimate the salinity and water temperature based on other conditions. Water temperature warms up along with air temperature, and rainfall changes salinity and amount of nutrients in the sea water.  Since weather records tend to be longer running and more consistent, the researchers could puzzle out the big picture.

Bad News for Kelp

Figure 3: An urchin barren forms when sea urchin populations increase and overgraze kelp, leaving large areas of the sea floor barren. Photo by John Turnbull.

The team found that from the 1930s to the 1980s, kelp covered 21-58% of the surveyed area in Port Phillip Bay. However, during the early 2000s kelp levels declined dramatically. This loss coincided with a roughly 1°C increase in air temperature from 1997 to 2009 and the most profound drought in that region in recent history, which resulted in nutrient-poor, saltier water. When a three-year lag was factored into kelp densities in the area, suggesting that kelp take a few years to respond to environmental conditions, the relationship between air temperature and rainfall matched. To make matters worse, sea urchin numbers shot up from 2005 to 2014, feasting on the remaining large algae in the area. The decline of kelp was so striking that during the last year of data, 2014, there was no kelp found in Port Phillip Bay. Carnell and Keough drew a clear conclusion: kelp began to decline, initially, due to high temperatures and little rainfall, and the hungry urchins wiped out the few remaining kelp.

These findings are alarming, because  kelp forms an essential habitat for coastal species. The study demonstrated how dramatically some ecosystems may collapse in response to climate change. As we continue to try to understand our changing planet with few long-term records available, Carnell and Keough show how important it is to use multiple sources of data to study an ecosystem in flux.

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