Eric W. Vetter

Population dynamics of a dense assemblage of marine detritivores

Accumulations of macrophyte detritus provide food and refuge for a dense (up to 3.5·106 ind. m−2) assemblage of amphipod and leptostracan crustaceans in a Southern California submarine canyon. The objective of this work was to determine how the physical environment and biological interactions limit these populations, that rarely, if ever, appear to be food limited. Preliminary observations suggested that the detritus-crustaceans would incur significant losses during the Winter, resulting from wave disturbance. Predation by dense schools of fishes associated with this habitat also appeared to be important. Laboratory and field experiments demonstrated that the detritus provided an effective refuge for the crustaceans. Predation rate of fishes upon detritus-associated crustaceans increased as the density of prey populations increased and/or as the thickness of the detritus mat was reduced (increasing number per m3 but not per m2). Density (m−2) of the mat crustaceans fluctuated seasonally, being greatest in the Winter and Spring following storms that eliminated large portions of the habitat. Few animals appeared to be directly eliminated by the storm disturbance. The important effect of storms was to reduce the quantity and quality of the detritus refuge by concentrating the crustaceans into smaller patches with less detrital cover. During the calm summer months the bacterium Beggiatoa sp. spread out over large portions of the detritus mat. Beneath the bacteria oxygen concentration was reduced, and infaunal density was two orders of magnitude lower than in unaffected portions of the mat. The summer increase in bacterial cover constituted a biological disturbance that functionally reduced the habitat area available to the mat fauna and left them more vulnerable to predation. Secondary production in the detrital mats is among the highest reported from natural environments. This is possible because the mat crustaceans are unable to graze down their food supply (detritus and associated microbes). The refuge provided by the detritus for large populations of invertebrates also is crucial; larger refugia translated into increased carrying capacity which allowed both greater population sizes and production, much of which became available to fishes. Most marine ecosystems are supported by allochthonous material that enters food webs via detritivores. Seafloor features that collect detritus constitute an important source of patchiness over a wide range of scales because detrital delivery is governed by physical processes. The La Jolla/Scripps Canyon system, by accumulating organic debris, provides a large food source from shallow to continental slope depths. This resource supports large numbers of fishes and presumably increases local production in higher trophic levels.

Results of international field experiment on a natural CO2 analogue

Publisher Summary This chapter highlights an International field experiment that was conducted in order to investigate a natural analogue of CO 2 , and its effects on the surrounding marine biology. This natural CO 2 analogue was investigated as a first order proxy for a purposeful CO 2 injection in order to determine potential effects such as depressed pH and hypercapnia on existing marine ecosystems. Trapping rates at Loihi were much higher than anticipated with a minimum of 1,100 amphipods recovered after a 24 hour deployment despite the loss of many animals through the funnel opening. On all dives the amphipods remained continuously active during the ascent to the surface until ambient temperatures of between 10 and 11°C were reached at a depth of about 300 m. From that point activity decreased rapidly until all appeared to be dead soon after the temperature exceeded 13°C at depths of about 250 m. These data indicate that the state of torpor which the majority of amphipods experienced following a 10-minute exposure to the vent fluids was due to the high CO 2 content and not the elevated temperature.