| Summary: | Great congregations of seabirds in coastal areas of the Southern Ocean result in delivery of nutrient-rich guano to coastal waters potentially enhancing productivity and supporting biodiversity in the region. The magnitude of the effects of seabird guano on phytoplankton production and community composition and the associated implications for ecosystem function and biogeochemical cycling are largely understudied. The overall goals of the present research project were to investigate effects of seabird guano on phytoplankton production and community structure in the Southern Ocean and to explore possible consequences of these effects for marine food webs. Several seabird guano-enrichment experiments on natural phytoplankton communities were carried out to address the research goals. These incubations were performed in four water masses, each with a distinct nutrient regime: Antarctic waters of the Ross Sea and sub-Antarctic waters offshore of the Otago Peninsula both having iron limitation of phytoplankton productivity in summer, the SubTropical Frontal Zone offshore of the Snares Islands which is generally micronutrient-replete, and in the island wake waters of the Snares Islands which have a high nutrient supply from land. In all of the experiments guano-stimulated increases of phytoplankton biomass were higher compared to phytoplankton responses in untreated control samples. As these findings were consistent across all water masses tested, similar general effects of guano are likely to extend across the Southern Ocean. Guano additions were compared to Fe and Macronutrients treatments (both added in quantities similar to those in the guano treatment) to shed light on which constituent(s) of guano is(are) responsible for the observed increases in phytoplankton biomass. The macronutrient component of guano influenced the phytoplankton response in the Sub-Tropical Frontal Zone. However, the response in the Macronutrients treatment was less prominent than in the Guano treatment suggesting synergetic effects of nutrients or the impact of another nutrient in seabird guano on phytoplankton production. In the SubAntarctic Zone, the response was similar between guano, Fe and macronutrients, providing an evidence for a possible co-limitation. Analysis of phytoplankton community composition demonstrated that seabird guano derived nutrients enhance growth of nano-sized phytoplankton, particularly prymnesiophytes. As nanophytoplankton are directly available to microzooplankton and/or copepods, the shift towards nanophytoplankton likely contributes to formation of a classical type food chain. In the Ross Sea guano-driven increases of phytoplankton biomass were attributed to diatoms that are also grazed on by copepods. Thus, within each of the water masses, guano fertilisation was found to promote a shift towards phytoplankton species that underpin a classical type marine food web, which supports high biomass of consumers (e.g. fish, seabirds) and enhances export of organic material from the euphotic zone. A simultaneous guano-driven increase of pico-sized phytoplankton biomass may enhance microbial recycling of nutrients within these food webs. Findings of the present study confirm that biological recycling of nutrients by seabirds supports marine primary production, and extends this effect to marine phytoplankton communities. Guano-driven responses of natural phytoplankton communities likely affect the structure of associated marine food webs and as a consequence ecosystem functioning. The results described here contributes to our understanding of the roles of higher trophic level species in accumulating and redistributing limiting nutrients in the Southern Ocean.
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