Seasonal variation in kelp phlorotannins in relation to grazer abundance and environmental variables in the Alaskan sublittoral zone

Abstract

Phlorotannins are common metabolites produced in kelps that can have deterrent functions against grazers. The factors dictating seasonal patterns of phlorotannin content in northeastern Pacific kelps are not well understood. This study assessed density and grazing of the gastropod Lacuna vincta on the annual canopy-forming kelp Nereocystis luetkeana and the perennial understory species Agarum clathratum, Saccharina latissima and S. groenlandica in Kachemak Bay, Alaska. In addition, we assessed seasonal patterns of environmental variables as possible drivers of phlorotannin concentrations. Phlorotannins occurred in all species, with overall lowest levels in N. luetkeana, and with different seasonal patterns among the four species. Lacuna vincta was most dense on N. luetkeana thalli in the summer and had highest grazing rates on this low-phlorotannin species. However, correlations between L. vincta density and phlorotannin content of each kelp species were not significant. Except for N. luetkeana, there were no correlations between phlorotannin levels and environmental variables. We suggest that kelp life history traits may be more important for phlorotannin patterns in these kelp species than grazers or environmental drivers.Key Words: environmental variables; grazers; kelps; life history; phlorotannins INTRODUCTIONKelps (Order Laminariales) form the basis of essential habitats in temperate coastal ecosystems because of their high productivity and function as spawning and nursery grounds for many invertebrate and fish species (Steneck et al. 2002). However, intense grazing can severely compromise kelp abundance and function (Scheibling et al. 1999). Mesograzers of the nearshore marine environment such as gastropods (Iken 1999, Granado andCaballero 2001) are capable of affecting the growth and reproductive abilities of individual macroalgal species (Dean et al. 1984, Dethier et al. 2005 and even the composition and density of macroalgal communities (Carney et al. 2005). Mesograzer habitat and food choice is often attributed to algal morphological characteristics like shape, size and tissue toughness (Littler and Littler 1980, Steneck and Watling 1982, Hay et al. 1994, and to chemical properties related to nutritive value (Cruz-Rivera and Hay 2003) and anti-herbivore defense (Paul et al. 2001, Amsler 2008.Kelps usually contain phlorotannins, phloroglucinolbased polymers present in most brown algae (Ragan and Glombitza 1986, Amsler andFairhead 2006), which have primary metabolic roles in wound healing (Lüder and Clayton 2004) and cell wall construction (Schoenwaelder and Clayton 1999). Phlorotannins also have been assigned secondary functions such as protection against UV radiation (Pavia et al. 1997), bacterial and fungal growth and fouling (Ragan and Glombitza 1986, Wikström and Pavia 2004 10 MATERIALS AND METHODS Study species and sitesField studies occurred off the western end of Hesketh Island (59°30.3′ N, 151°31.8′ W) within Kachemak Bay, south-central Alaska. The study site was located on exposed shores ap...

Ocean acidification and food limitation combine to suppress herbivory by the gastropod Lacuna vincta

“…For all grazing experiments, 8 Lacuna (~2 mm) were added to each vessel. The number of Lacuna used mimicked densities found on Ulva assemblages at the collection site (0.5−1 grazer cm −2 ) and reported in the literature (Chenelot & Konar 2007, Dubois & Iken 2012.…”

An Assessment of the Influence of Host Species, Age, and Thallus Part on Kelp-Associated Diatoms

“…It has been suggested that macro-and micro-topographically uniform smooth blades of large kelps may not be a preferred attachment surface for many diatom taxa [8,13,49], while epithelial cell sloughing that is most likely common in long-lived kelps [18,19,21] would effectively limit the number of diatoms developing on the thallus. Moreover, production of secondary metabolites is used by various kelp species to deter herbivores [53][54][55]. However, the bioactive compounds excreted by macroalgae may affect not only grazers but also the microalgal biofoulers-both directly and indirectly.…”

Hypoxia and Acidification, Individually and in Combination, Disrupt Herbivory and Reduce Survivorship of the Gastropod, Lacuna vincta

“…Based on the amount of U. rigida used in experiments (∼8 -9 cm 2 per sample), the appropriate number of L. vincta was added. The number of L. vincta used mimicked densities found on U. rigida at the collection site (0.5 -1 grazer cm −2 ) and reported in the literature (Chenelot and Konar, 2007;Dubois and Iken, 2012;Young et al, 2019).…”