On-line version ISSN 0717-7178
Investig. mar. vol.30 no.1 suppl.Symp Valparaíso Aug. 2002
Biomass and Bioturbation Responses of
Macrobenthos during El Niño 1997-
1998 in Continental Margin Sediments
off the Southeastern Pacific
1Instituto del Mar del Perú, Callao, Perú, P.O. Box 22,
Callao, Perú, E-mail: email@example.com,
2Centro de Investigaciones Oceanográficas en el
Pacifico Sur-Oriental, Universidad de Concepción,
Casilla 160-C, Concepción 3, Chile,
3Integrative Oceanography Division, Scripps Institution
of Oceanography, La Jolla, CA 92093-0218, USA,
Introduction & Objectives
The 1997/98 El Niño (EN) was one of the strongest El Niños of the past century. Positive sea-surface temperature anomalies along the Southwestern coast of South America were recorded from March/April 1997 on and the event concluded in June of 1998. This study is focused on the benthic biomass and bioturbation in shelf and upper slope sediments off central Chile (36°S) and off central Peru (12°S). During non-El Niño years, these settings are subjected to severe hypoxic conditions (<0.5 ml L-1) seasonally (central Chile) or permanently (central Peru). Our objectives are: i) To compare environmental perturbations associated with the 1997/98 EN on the benthic subsystems of the continental margin off central Chile and off central Peru; ii) To assess the benthic biomass and potential bioturbation responses in both areas during EN. Bioturbation rates were estimated from chl a vertical profiles in the sediment column off central Chile.
Results & Discussion
In both areas, during the 1997/98 EN the organic matter flux toward the benthos and bottom hypoxia were reduced, though there were differences in the magnitude of the perturbations. Off central Chile, bottom water oxygenation was evident during the spring/summer upwelling period, especially in shallow areas (< 60 m) where oxygen content records varied from 1 to 3 ml L-1 . Modelled chl a fluxes to the sediments indicated a significant reduction of fresh organic input despite unchanged total organic carbon (TOC) content in the surface sediments. Off central Peru, pulses of transport of oxygen-rich subsurface waters deepened the upper boundary of the oxygen minimum zone from 50-100 m down to 200 m depth; values up to 3 ml L-1 were measured at 90 m depth. Chl a fluxes were not assessed but a significant reduction of TOC content at the surface sediments was recorded.
Off central Chile, benthic responses to EN varied in sign, magnitude and/or persistence in relation to depth and bottom topography. In the semi-enclosed Bay of Concepción, a massive recolonization by macrofauna of the previously nearly defaunated sulphidic sediments occurred from August 1997 on, lasting until the end of 1998. Following the EN nearly anoxic conditions rapidly developed over the bottom and sediments became sulphidic (as before the EN). At this time the macrofauna experienced significant mortality. Burrowing and bioturbation varied in direct relation to the temporal change of macrofaunal biomass. Longer lasting responses were observed in the open shelf. Here total macrofaunal biomass tended to decrease, as a result of impoverished populations of tube-dwelling, interface -feeders. Nevertheless, a partial biomass increase of subsurface deposit-feeders and burrowing omnivores or predators was observed, especially in deeper shelf areas. Despite declining macrofaunal biomass, during the EN bioturbation increased in the open shelf, due largely to the alteration of functional guilds and to deeper animal vertical distribution. Nearly one year after the end of EN, interface-feeders had not recovered their initial population levels and macrofaunal biomass remained lower than at the EN onset, but bioturbation rates decreased to early EN levels (Fig. 1).
Fig. 1 Temporal change of bioturbation rates based on chl a profiles at four study sites off Concepcion (36°S), central Chile during the 1997/98 EN. Periods: I=early EN, II=late EN, III=after EN. Three cases are shown: i) with constant chl a degradation rate k; ii) with constant k and excluding possible non `steady state' cases; and iii) with variable k following Arrhenius law and excluding possible non `steady state' cases.
Off central Peru, as soon as oxygenation took place, a rapid recolonization by macrofauna was observed and bacterial mats disappeared in the open shelf sediments, which were previously devoid of macrofauna and covered by Thioploca mats. Despite the `normal' hypoxic conditions that developed after the EN, large-bodied macrofaunal populations survived until mid-1999. Burrowing polychaete omnivore/predators increased their numbers and biomass during EN, as well as large-bodied and deep burrowing nemerteans and enteropneusts. Similarly, sediment cores collected from previously anoxic sediments on the upper slope (300 m) appeared to have been recently heavily colonized by a burrowing, gutless oligochaete by early 1998. Thus, oxygenation and organic matter flux changes associated with the 1997-98 El Niño appears to have led to in creased bioturbation in both the Chilean and Peruvian systems. Our results suggest that during non-EN years, there are large fluxes of phytoplanktonic carbon to the seafloor, and most of the sediment microbial remineralization is restricted to the uppermost sediments. In contrast, during the less productive and less hypoxic EN years, bioturbation enhancement would deepen the microbial activity to include an older organic material. This may potentially result in higher benthic carbon respiration rates relative to the carbon input to the benthos.