the community composition and discernible shifts in composition. The pictures reveal
that 10 invariant species, all echinoderms, constitute 99% of crawlers on the sediment
surface: eight sea cumbers (holothurians), one urchin (echinoid), and one brittle star
(actually all ophiuroids lumped). In 37 surveys, the brittle stars always ranked one or
two. With one outstanding exception, the rank-order shifts were about those expected
from sampling variation. That exception was that the usually dominant cucumber,
Elpidia minutissima, shifted in 2001–2004 pictures (Plate 14.4)) from consistently
ranking number one or two to number 10. In fact, it essentially disappeared. In the
same interval, the urchin Echinocrepis rostrata shifted in both relative abundance,
moving up from fifth rank to third, and increasing by about 10-fold in population
density. The urchin increase clearly involved a recruitment event, since median
individual size dropped substantially. Ruhl’s (2008) cluster analysis of the varying
proportions of these 10 species from 1989 to 2004 showed convincing grouping of the
sampling dates into eras, of which the shift in 2001, persisting to 2004, is the most
distinctive. Ruhl sees a less-convincing correspondence to blocks of time in the
Station M food-input data and approximations (Smith et al. 2001, 2006), and suggests
that the changed composition of 2001 and later years could have been a response to
the severe 1998 El Niño. Changes in macrofauna counted from box cores occurred but
were more chaotic (Smith et al. 2009, their Fig. 4).
An Atlantic Time-Series
(^) A similar but less protracted time-series of deep-sea trophic studies was conducted by
British ecologists on the Porcupine Abyssal Plain: Station PAP at 4800 m in the North
Atlantic subpolar gyre at 49°N, 16.5°E, due west of Brittany, centered between the
European slope and the Mid-Atlantic Ridge. Results for 1996 through 2004 are
published (Lampitt et al. 2010a). POC flux (Fig. 14.29) was measured with a conical
0.5 m^2 trap at 3100 m with collecting cups changed every two weeks in intervals of
high flux, every eight weeks in periods of low flux. Satellite chlorophyll estimates,
averaged for a 200 km radius circle around PAP, became available for comparison
from October 1997. Chlorophyll (and thus primary productivity) and flux varied on
roughly similar schedules (Lampitt 2010b). However, in some years, 1999 and 2001,
peak flux lagged peak chlorophyll by several months, while in other years no lag was
obvious. A single year, 2001, had much greater flux, although chlorophyll levels in
that year were not markedly more than in 1998 and 1999. In summers of 2002 through
2005, chlorophyll was low compared to 1998 to 2001, and flux was also low (as it
was in 1998 and – save for one sample – 1999). The late-summer pulses of POC flux
were actually periods of greater ratios of POC flux to mineral flux: there was
relatively low flux of particulate calcium carbonate and opal. This aligns with doubts
in the literature that mineral ballasting is a particularly significant determinant of rates
of food supply to the deep sea (reviewed by Passow & De La Rocha 2006).