Context dependency of top-down and bottom-up effects in a Northern Australian tropical river

Erica Garcia, Simon Townsend, Michael Douglas

    Research output: Contribution to journalArticleResearchpeer-review

    Abstract

    Top-down and bottom-up forces (consumer and resource limitation, respectively) influence biomass of primary producers and primary consumers in natural food webs. Few investigators experimentally examine both in concert, especially in the tropics. Tropical systems probably are more sensitive than temperate systems to eutrophication and other disturbances, such as destruction of riparian canopy cover, because of wide windows of ecological opportunity. We experimentally examined the relative importance of top-down and bottom-up effects at 2 sites in the Edith River in the Australian wet-dry tropics during baseflow conditions. We used large fishexclusion cages to examine top-down effects of fish, and we used nutrient-diffusing substrates to manipulate bottom-up effects of nutrients. Macroinvertebrates were significantly more abundant in fish-exclusion cages than in open control areas at both sites. At 1 site, chlorophyll a on rock substrate was 1.5�greater and ash-free dry mass was 1.4�greater in fish-exclusion than in open cages. Chlorophyll a was >3�greater in the high-N and -P nutrient treatment than in the unamended control at both sites. Top-down effects on the algae on nutrientdiffusing substrates were detected at only 1 site. Bottom-up factors seem to be relatively more important in controlling primary producer biomass compared to top-down factors, with no evidence of a trophic cascade. Bottomup and top-down effects on macroinvertebrates were strong and consistent, whereas top-down effects on algae were context dependent and varied depending on benthic habitat, current speeds, and community structure. These results indicated spatial variation at the scale of tens of meters.
    Original languageEnglish
    Pages (from-to)679-690
    Number of pages12
    JournalFreshwater Science
    Volume34
    Issue number2
    DOIs
    Publication statusPublished - 2015

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    algae
    cages
    macroinvertebrates
    rivers
    nutrients
    fish
    river
    chlorophyll
    natural foods
    biomass
    substrate
    macroinvertebrate
    eutrophication
    food webs
    spatial variation
    nutrient
    tropics
    chlorophyll a
    community structure
    rocks

    Cite this

    Garcia, Erica ; Townsend, Simon ; Douglas, Michael. / Context dependency of top-down and bottom-up effects in a Northern Australian tropical river. In: Freshwater Science. 2015 ; Vol. 34, No. 2. pp. 679-690.
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    Context dependency of top-down and bottom-up effects in a Northern Australian tropical river. / Garcia, Erica; Townsend, Simon; Douglas, Michael.

    In: Freshwater Science, Vol. 34, No. 2, 2015, p. 679-690.

    Research output: Contribution to journalArticleResearchpeer-review

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