L
Journal of Experimental Marine Biology and Ecology 244 2000 285–296
www.elsevier.nl locate jembe
Clearance capacity of Electra bellula Bryozoa in seagrass meadows of Western Australia
Dennis Lisbjerg , Jens Kjerulf Petersen
National Environmental Research Institute , P.O. Box 358, Frederiksborgvej 399, DK-4000 Roskilde,
Denmark Received 15 December 1998; received in revised form 23 March 1999; accepted 6 October 1999
Abstract
Filtration rates were measured as the clearance of algal cells Rhodomonas sp. in the laboratory for the bryozoan Electra bellula Hincks. The colony clearance rates were related to both total
and specific active area of the colony, and a closer correlation was obtained when relating clearance to specific area. All results were therefore related to specific colony area. On average
49 of total colony area had active zooids. Clearance rates were measured at temperatures ranging from 16 to 248C. Maximum specific clearance rates F
were from the 2–3 replicates with the
max
highest specific clearance rates out of 3–8 experiments performed with each colony. F varied
max 21
22 21
22 21
22
from 69 ml h cm
at 168C to 107 ml h cm
at 248C. Highest F of 115 ml h
cm was
max 22
measured at 208C. Dry weight DW related to total area by W 5 5.15 mg cm
and ash-free
DW 22
21 21
21 21
dry weight AFDW by W 5 1.15 mg cm
. F 5 9.5 l h
g DW and 43 l h
g
AFDW max
AFDW at 228C. The clearance capacity of bryozoan communities in seagrass meadows of Western Australia is estimated by use of these results.
2000 Elsevier Science B.V. All rights reserved.
Keywords : Bryozoans; Clearance rate; Electra bellula; Filtration rate; Temperature effect
1. Introduction
In coastal marine areas macro suspension-feeders such as ascidians, polychaetes, bivalves, and sponges form dense populations. Due to their large filtration capacity,
these taxa may potentially process major parts of the water column on a daily basis in shallow areas and affect the levels of the phytoplankton and other suspended particles in
the water column e.g. Doering and Oviatt, 1986; Loo and Rosenberg, 1989; Petersen
Corresponding author. Tel.: 1 45-46-301-295; fax: 1 45-46-301-211. E-mail address
: dlidmu.dk D. Lisbjerg 0022-0981 00 – see front matter
2000 Elsevier Science B.V. All rights reserved.
P I I : S 0 0 2 2 - 0 9 8 1 9 9 0 0 1 4 7 - 1
286 D
. Lisbjerg, J.K. Petersen J. Exp. Mar. Biol. Ecol. 244 2000 285 –296
˚ and Riisgard, 1992; Cloern, 1996. Lemmens et al. 1996a estimated the biomass and
filtering capacity of several benthic groups in seagrass meadows of Western Australia, and concluded that ascidians and polychaetes are the two groups of macrofauna that
contribute most to community filtering capacity. These groups have been studied further Clapin, 1996; Lemmens et al., 1996b; Lemmens and Petersen, in prep.. Because of
their minute size, epifauna such as bryozoans, spirobids, and cirripeds have often been neglected in community analysis assessing the flux between the water column and
benthic communities. However, Lemmens et al. 1996a found that small epifaunal suspension-feeders also contribute considerably to the total filtering capacity and
especially bryozoans may be of significance.
Literature concerning the role of bryozoans in an ecological perspective is very limited. In the few early works by Menon 1974 and Bullivant 1968 experiments were
performed in order to measure bryozoan filtration rates at different temperatures and different food concentrations. They obtained filtration rates measured as the clearance
i.e. the reduction in the number of algae in a beaker containing a colony. In several later studies e.g. Strathmann, 1982; Best and Thorpe, 1986, 1994; Sanderson et al., 1994;
˚ ´
Riisgard and Manrıquez, 1997 pumping rates have been estimated from particle velocity within the lophophores and the cross-sectional area of the lophophores. Best and
Thorpe 1983 calculated feeding rates by timing the frequency of pharynx emptying, assuming equal size of the bolus formed before emptying, and estimating the volume
and number of cells the bolus contains.
In order to estimate the clearance capacity of bryozoans in ecological terms, individual clearance rate and population densities are needed, but also morphological
and life cycle variations within the bryozoan group is important; all species are polymorphic to some extent, implying that some zooids within a colony might be
specialized as e.g. some sort of defence organs avicularia or vibracularia, spinozooids
´ or involved in reproduction, forming distinct male female zooids or ovicells Silen,
1977. Because of this partitioning, only a part of the colony will contain actively feeding zooids. Also, in bryozoans, the feeding zooids perform a cycle of polypide
regression brown body formation and regeneration, which is dependent on food availability Bayer et al., 1994. Due to this cycling, an even lesser part of the colony
may be actively feeding at any time and this must be addressed when estimating colony clearance capacity. In the present study clearance rate of a bryozoan species common to
the seagrass meadows of the southern hemisphere is measured in order to elucidate bryozoan colony clearance as a function of temperature. Emphasis is on bryozoan
clearance capacity and its ecological implications.
2. Materials and methods
