Are Pfiesteria species toxicogenic? Evidence against production of ichthyotoxins by Pfiesteria shumwayae
In general, hypoxia (< 2 mg O-2 l(-1)) is detrimental to marine food webs because of faunal declines associated with persistent, severely low oxygen. However, transfer of benthic production to higher trophic levels could be facilitated under hypoxia if infauna migrate to shallower burial depths, increasing their availability to predators. A series of outdoor mesocosm and laboratory experiments were conducted in 3 years (1999 to 200 1) to quantify the effects of hypoxia upon (1) predation by the blue crab Callinectes sapidus on the Baltic clam Macoma balthica, a biomass dominant in Chesapeake Bay, and (2) burial depth and survival of M, balthica. For the predation experiments, 12 M. balthica were transplanted into replicate 1200 1 mesocosms at 48 clams m(-2) under normoxia (>8 mg O-2 l(-1)) or low dissolved oxygen (low DO; <2 mg O-2>l(-1)) either without or with a male blue crab that was allowed 2 d to feed upon the clams. Predation-induced proportional mortality of clams was significantly lower under low DO than under normoxia in all 3 years. Thus, under short-term hypoxia, both crab feeding efficiency and trophic transfer from M balthica to blue crabs were reduced. Changes in clam burial depth due to oxygen levels was determined by establishing normoxic and low DO treatments in replicate 208 1 aquaria in 2 years. Burial depth after exposure to oxygen treatments for 48 h did not differ as a function of oxygen level. None of the clams died after 2 d of low DO, 27 % died after -6 d, and 90 % died after 21 d. Short-lived hypoxia therefore reduces the ability of crabs to forage upon clams efficiently and increases clam survival, whereas long-term hypoxia may increase the availability of clam prey to predators through mortality and movement to the surface. Thus, short-lived hypoxia is likely to reduce the transfer of benthic prey to higher trophic levels, although longer-term exposure may increase transfer.