The dynamics of kelp forests

Cathy Pfister (University of Chicago) has recently had a paper published on kelp forests in the Northeast Pacific Ocean. She tells us more about her paper below…


In the decade or so before WWI, Germany had a near monopoly on an essential resource: potash mines that supplied fertilizer (as well as gunpowder) globally.

The possibility that kelp forests off the coast of the US could provide this resource prompted the US Congress to fund an expedition to quantify kelp beds along the sinuous and vast US West coast from Alaska to California. It is from this 1911-1912 survey that we can now make inferences about the health and persistence of Washington state kelp beds.

In our paper, we use this remarkable US Department of Agriculture survey as well as 26 years of aerial surveys by the Washington Department of Natural Resources to show that kelp beds in the western Strait of Juan de Fuca and the outer coast are persistent, while kelp forests eastward toward increasing human population density are lower in abundance compared with a century ago.

Kelpbeds_WA_2006

The Olympic Peninsula, Washington

There are other key findings here that reveal the dynamics of foundation species in our ocean ecosystems. Our analyses reveal that some kelp beds, while persistent over decadal scales, can fluctuate greatly from year to year. While fluctuations in abundance can be normal for natural populations, they also have genetic consequences and can put populations at risk.

Another insight from these data is that the 2 species that compose kelp beds, the annual bull kelp Nereocystis luetkeana and the perennial giant kelp Macrocystis pyrifera, have positively correlated dynamics: a good year for one species is also good for the other. This reveals a vulnerability of these populations, because a poor year for one species is also a poor year for the other. Thus, the ecological concept of ‘complementarity’, where productivity might be buffered by differential species responses to environmental variation may not apply here.

Pfister_close_up of 2 kelp forest species

Nereocystis luetkeana and Macrocystis pyrifera (Photo: C. Pfister)

As our oceans change, a key question is how these giant primary producers in the ocean are affected by the change. Kelp forest dynamics correlated inversely with the Pacific Decadal Oscillation and the Oceanic Niño Index in our study, suggesting that negative phases of the PDO that are associated with colder water were beneficial to kelp growth. A 93-year temperature record in the region revealed a 0.72oC increase over this period.

Our future analyses need to focus not only on kelp forest responses to these changing climate variables, but also on kelp patterns in Puget Sound. The responsiveness of kelp to climate indices, coupled with ocean change and increasing human population in these areas warns us that kelp forests deserve further attention.

Cathy Pfister, University of Chicago, USA


Read the full paper online: The dynamics of Kelp Forests in the Northeast Pacific Ocean and the relationship with environmental drivers

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