Seasonal dynamics of megafauna on the deep West Antarctic Peninsula shelf in response to variable phytodetrital influx

Abstract

The deep West Antarctic Peninsula (WAP) shelf is characterized by intense deposition of phytodetritus during spring/summer months, while very little food material reaches the seafloor during winter. The response of the shelf benthic megafauna to this highly variable food supply is still poorly understood. In order to characterize the deposition of phytodetritus and the megabenthic community response, we deployed a seafloor time-lapse camera at approximately 590 m depth on the mid WAP shelf west of Anvers Island for 15 months. Seafloor photographs were taken at intervals of 12 or 24 h nearly continuously from 9 December 1999 (austral winter) to 20 March 2001 (summer) and analysed for phytodetritus deposition and megafaunal dynamics. Seafloor images indicated a marked seasonal arrival of greenish phytodetritus, with large interannual and seasonal variability in the coverage of depositing phytodetrital particles. The surface-deposit-feeding elasipod holothurians Protelpidia murrayi and Peniagone vignoni dominated the epibenthic megafauna throughout the year, frequently constituting more than 80% of the megafaunal abundance, attaining total densities of up to 2.4 individuals m(-2). Elasipod abundances were significantly higher in summer than winter. During summer periods of high phytodetrital flux, Pr. murrayi produced faecal casts at higher rates, indicating intensified population-level feeding activity. In March-June 2000, faecal casts lasted longest, suggesting lower horizontal bioturbation activity during autumn-winter. Our data indicate that the Pr. murrayi population increases its feeding rates in response to increasing amounts and/or lability of organic matter on the sediment surface. Assuming that this species feeds on the top millimetre of the sediment, we estimate that, during periods of high phytodetrital flux, the Pr. murrayi population reworks one square metre of sediment surface in approximately 287 days. We suggest that Pr. murrayi is an important species for organic-carbon recycling on the deep WAP shelf, controlling the availability of deposited labile phytodetritus to the broader shelf benthic community.

Keywords: Antarctic Peninsula; benthos; deposit feeding; elasipod holothurians; megafauna; time-lapse photography.

Figure 1.

Location of the three stations (A–C) sampled on the West Antarctic Peninsula shelf during the FOODBANCS project. The time-lapse camera system was deployed at the mid-shelf station (point B).

Figure 2.

Seafloor photographs taken at four different periods in the West Antarctic Peninsula (WAP) shelf Station B. (a) Picture taken in the November 1999–March 2000 period. Note the presence of a thin layer of phytodetritus at the seafloor. (b) March 2000–June 2000. During this period, no phytodetritus was found. (c) In June 2000–October 2000, no phytodetritus layer was observed. (d) November 2000–March 2001. Dense carpets of phytodetritus (approx. 2 cm thick) were found over the seafloor. Inset: faecal casts of the two main elasipod holothurians on the WAP shelf: (e) Pr. murrayi and (f) Pe. vignoni. The scale bar of the larger picture represents 20 cm. In the inset, the scale bar is 2 cm.

Figure 3.

Mean epibenthic megafaunal density (individuals m⁻²) measured over the study period. Vertical lines are ±1 s.e.

Figure 4.

Numerical density and size of elasipod holothurians in time-lapse photographs. Bars represent numerical density; triangles and stars are mean lengths of the elasipods Pr. murrayi and Pe. vignoni, respectively. Vertical lines are ±1 s.e.

Figure 5.

Mean number of faecal casts deposited on the seafloor by the holothurian Pr. murrayi pooled per sampling period. The right Y -axis represents scores for phytodetritus coverage of the seafloor as follows: no greenish, flocculent phytodetritus visible at the seafloor (score=0); diffuse, greenish, flocculent material visible in some areas but bioturbation traces readily visible (score 1); much of the seafloor (50–90%) covered with greenish phytodetritus, with bioturbation traces partially filled (score=2); more than 90% phytodetritus cover (score=3) [9]. Score 4 represents dense, up to 2 cm thick phytodetritus carpets. Vertical lines are ±1 s.e.

Figure 6.

Mean faecal-cast volumetric production rate per individual (a) and population-level faecal-cast production rate (b) for the holothurian Pr. murrayi. Data were pooled for each sampling period. Also shown in (b) is the POC flux in the study area collected during the same period by Smith et al. [9]. Vertical lines are ±1 s.e.

Figure 7.

Protelpidia murrayi population-level sediment reworking times for the top millimetre layer of sediment on the WAP shelf. POC flux data were collected for the same study area and period by Smith et al. [9].