Effects of ultraviolet-B enhancement on marine trophic levels in a stratified coastal system

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We examined the effects of enhanced UV-B radiation (relative to ambient) on marine trophic levels inhabiting a stratified coastal ecosystem, using living models (13000 liter marine enclosures) of a temperate estuarine water column. The experiment was carried out in June and July 1994 on a plankton community drawn from lower Narragansett Bay, Rhode Island, USA. The effects of altered UV-B radiation (elevated 50% over ambient, tenfold DNA-weighted) on three trophic strata: the primary producers (photosynthetic algae), primary herbivores (copepods), and fish eggs and larvae (Anchoa mitchilli Cuvier and Valenciennes) were examined. The goal was to determine if UV-B-induced alterations at the base of the food chain had impacts on other elements of the trophic web. Phytoplankton abundance (P = 0.02) and biomass (P = 0.007) were significantly reduced in UV-B-enhanced treatments, above but not below the thermocline (2.25 m), during the second week of the study. Copepod nauplii were significantly less abundant in UV-B-enhanced mesocosms than in control treatments during the third and fourth weeks of the study (P = 0.01). A portion of the impact on nauplii may be a result of alterations at the base of the food web. The greater mortality of nauplii in UV-B-enhanced systems did not translate to reduced abundance of copepodite (P = 0.83) or adult (P = 0.29) copepods. No significant effects were observed for microzooplankton (P = 0.15). Neither the mortality rates nor the growth rates of larval anchovy were significantly affected by the experimental increase in UV-B (P > 0.05). Despite the tenfold increase in biologically damaging UV-B, effects were not seen at higher trophic levels, most likely because of the rapid extinction of UV-B in the highly colored coastal water.

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Marine Biology