IMPACTS OF MESOGRAZERS ON EPIPHYTE AND ENDOPHYTE GROWTH ASSOCIATED WITH CHEMICALLY DEFENDED MACROALGE FROM THE WESTERN ANTARCTIC PENINSULA: A MESOCOSM EXPERIMENT¹

Abstract

It has been hypothesized that the extensive mesograzer community along the western Antarctic Peninsula regulates epiphytic algae as well as emergent filaments from endophytic species. Should grazing limit growth of fouling or potentially pathogenic microphytes, then Antarctic macrophytes may actually benefit from the remarkably high densities of mesograzer amphipods that occur in these waters. Although initially counterintuitive, the negative impacts of epi/endophyte fouling may outweigh stresses caused by limited amphipod grazing on chemically defended macrophytes by reducing stress from endo/epiphyte biomass. If so, then alleviating mesograzing stress should result in significant increases in endo/epiphytic biomass. To test this hypothesis, a mesocosm experiment was conducted. Individuals representing four common species of Antarctic macroalgae were placed in flow-through seawater mesocosms. Amphipods were added to five mesocosms at simulated natural densities, while the other five remained herbivore free. At the end of 7 weeks, endo/epiphytic growth on individual macrophytes was quantified. Most species of macroalgae demonstrated noticeably higher instances of endophyte coverage, epiphytic diversity, and diatom colonization in consumer-free mesocosms than in the presence of amphipods. These data suggest that macroalgae along the western Antarctic Peninsula rely on grazers to control populations of potentially harmful epiphytes. We hypothesize that the chemically defended macroalgal flora lives in mutualism with high densities of mesograzers, providing amphipods with shelter from predation while continually being cleaned of potentially harmful endo/epiphytes.

Algicidal activity and potential antifouling defenses in macroalgae from the western Antarctic Peninsula including probable synergistic effects of multiple compounds

“…Another feature of the macroalgal assemblage is a relative lack of biofouling by microscopic or fi lamentous algae (Peters 2003, Amsler et al 2009b. A primary reason for this is probably grazing by high densities of amphipods and other small herbivores commonly associated with the macroalgae (Peters 2003, Amsler et al 2009b, Aumack et al 2011. However, some macroalgal species remain relatively free of diatom biofouling in the absence of these grazers (Aumack et al 2011 ), suggesting the possibility of macroalgal chemical defenses against diatoms in addition to those known for resistance to herbivores.…”

Tolerance and sequestration of macroalgal chemical defenses by an Antarctic amphipod: a ‘cheater’ among mutualists

“…However, filamentous algal endophytes growing within the larger, chemically defended macroalgae are unusually common (Peters 2003, Amsler et al 2009b). Peters (2003) hypothesized that the high densities of macroalgalassociated amphipods in these Antarctic communities were preventing the growth of free-living filaments while selecting for an endophytic growth form, and this has been supported by experimental field, laboratory, and mesocosm studies (Amsler et al 2009b, Aumack et al 2011b. Consequently, the dense assemblage of chemically defended Antarctic macroalgae benefits from the dense assemblage of amphipods associated with it because the amphipods limit the growth of both unicellular and filamentous epibionts on the macroalgae.…”

“…Other prominent macroalgal-associated amphipods in this community are thought to be in a communitywide mutualistic relationship with the dominant, chemically defended macroalgae. They benefit from an associational defense from fish predation (Zamzow et al 2010) while the macroalgae benefit from a reduction of epiphytic diatoms and filamentous algae as well as emergent filaments of algal endophytes (Amsler et al 2009b, Aumack et al 2011b. P. fissicauda would still benefit from the associational defense and may also consume some epiphytic diatoms and filamentous macroalgae (although its stable carbon isotope signature suggests that any such consumption is likely minimal; Aumack 2010).…”

“…Although these Antarctic amphipods do not appear to consume most host macroalgae, they consume large quantities of epiphytic diatoms (Aumack 2010, Aumack et al 2011b) which almost certainly benefits their hosts. Many studies throughout the world have shown that amphipods and other mesograzers can benefit their host macrophytes in this manner by limiting the growth of epiphytic microalgae and/or filamentous algae, which can compete with the macrophytes for light and nutrients (e.g.…”

Antarctic crustacean grazer assemblages exhibit resistance following exposure to decreased pH

“…Amsler et al (2014) review how these grazer assemblages play an important role in regulating benthic algal growth in this region. Aumack et al (2011b) noted that these grazer assemblages participate in a mutualistic relationship with their chemically defended macroalgal hosts, in which grazers receive refuge from predation, and in turn consume potentially detrimental epiphytic diatoms and other epiphytes from their macroalgal hosts.…”