Kelp: It’s whats for dinner (or where you live)

Simonson, E. J., Scheibling, R. E., & Metaxas, A. (2015). Kelp in hot water: II. Effects of warming seawater temperature on kelp quality as a food source and settlement substrate. Mar Ecol Prog Ser, 537, 105-119.  doi: 10.3354/meps11421

Last month, you may have read our post about Erika Simonson et al.’s work on how kelp in Nova Scotia is responding to warming waters. Simonson and her colleagues took kelp samples from the dense groves off the coast and grew them in a laboratory at temperatures representing predicted future water conditions. They found that kelp tissue degrades more rapidly at higher temperatures. The damaged tissue growing in warmer water could presage a wholesale community shift from leafy kelp fronds to mats of turf algae. Besides being less photogenic, turf algae are also less productive than kelp and would not support the diverse, highly productive ecosystems that live around the kelp.

Figure 1 - Two snails on a kelp leaf. The bottom snail is Lacuna vincta (photo: Asbjørn Hansen, Creative Commons License)
Figure 1 – Two snails on a kelp leaf. The bottom snail is Lacuna vincta (photo: Asbjørn Hansen, Creative Commons License)

But Simonson was not content to just consider the direct effect of warmer water on kelp. Indirect effects on the ecosystem, such as changes in the food web, might compound the primary environmental impact to the kelp. Simonson considered two possible community changes that could result from altered kelp tissue: First, whether Lacuna vincta, a native sea snail, finds the damaged kelp more or less appetizing (fig. 1). And, second, if Membranipora membranacea, a kind of invasive moss, found the damaged kelp more suitable for settlement (fig. 2).

Sea snails, like your kid brother, are picky eaters; they prefer to graze on kelp that contains a lot of nitrogen and low levels of a substance called phlorotannins. Phlorotannins are chemicals that kelp produce in their cell wall to protect themselves from UV light and grazers. The kelp grown in warm water might accumulate different levels of nitrogen or pholortannins, which, in turn, might encourage or discourage the snails from eating it.

Elevated levels of phlortannins might also serve to protect kelp from the invasive moss, M. membranacea. While snails eat the kelp, M. membranacea likes to live on it. More phlorotannins could keep the moss from settling on the kelp leaves in the first place. The warm water kelp tissue may also be less appealing simply because the damaged leaves have less surface area.

For these experiments, Simonson and her group had to collect kelp, snails, and moss. The team got samples of the kelp and the snails by SCUBA diving off the coast of Nova Scotia. M. membranacea was collected in its larval stage from plankton net samples taken near-by. Back at the lab, the kelp was grown in two temperature treatments: 11° and 21° C. The cooler water represents the ambient current temperature while the warmer simulates predicted maximum conditions in the next 100 years. The snails and moss were kept in separate tanks of ambient seawater.

Figure 2 – M. membranacea is the white net structure growing on the kelp leaf. This is taken after the moss settles on the plant. It starts life as a drifting plankton until it finds someplace to settle (photo: Eugene van der Pijil via Wikipedia, Creative Commons License)

To test if these organisms prefer warm water kelp to cool water kelp, Simonson conducted weekly “choice, no-choice” experiments. The procedure is designed to see which type of kelp the snails and moss prefer. Kelp tissue was trimmed from plants grown in both the 11° and 21° C water and presented to the animals in different combinations. For the no-choice tests, the snails and moss were given just samples from the 11o or the 21o C kelp. The choice test entailed giving the test subjects tissue from each temperature tank.

The scientists monitored the snails’ growth rate and egg production to see if the they preferred kelp from either treatment. Growth rate was recorded as the change in length of the shell. Egg production was logged as the number of new eggs counted weekly from the specimen. They likewise counted how many M. membranacea larvae settled on the kelp leaves in each experimental set-up. In addition to tracking the feeding/settling behavior, Simonson also measured the chemical properties of the kelp to record changes in its nitrogen and phlorotannin content. The team repeated the whole experiment three times from beginning to end.

Simonson found that the snails and moss had no discernable preference for kelp grown at either temperature. The snails ate everything regardless of how it was grown. The snails seemed to eat more of the kelp pretreated in 21°C water in both the choice and no-choice tests. Simonson suggests this is because snails prefer softer food, like the damaged kelp tissue, regardless of nutritional content. Similarly, M. membranacea settled in equal quantities on kelp grown under both conditions.

The chemical data showed that the water conditions did not have a significant effect on the amount of nitrogen or phlorotannins produced by the kelp. This explains the behavior of both organisms. The snails were not attracted by higher levels of nitrogen in either kind of kelp, nor were they repelled by higher levels of pholortannins. M. membranacea also did not have to worry about different levels of pholortannins and settled wherever it pleased.

The authors found that neither grazing by snails nor settlement by the moss was enhanced on kelp grown at warmer temperatures. Simonson argues, however, that these secondary effects will act additively to increase biomass loss of kelp in a warming ocean. Snails will keep eating kelp, the moss will keep growing on it, and all while the tissue itself degrades in warmer water. This triple threat of tissue damage will cause more kelp to die and get ripped up in storm events. Eventually kelp groves will drastically recede, impacting habitat availability, community productivity, and the export of kelp detritus (i.e. food) to coastal and deepwater environments.

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