Responses In feeding behaviour to wide vanatlons in the amount and composition of natural seston were studled In the green-lipped (GreenshellT\'] mussel Perna canaliculus of standard 58 + 2 mm shell length Nutnent acquisition wds pnmanlv regulated by the rate that mussels cleared water of part~cles (I h ' I , which vaned In hyperbol~c relation with the abundance of chlorophyll-rich organics, lather than with the ren~alning organic 01 inorganic components of ava~lable 5eston Maximal clearance rates of about 15 1 h ' g ' consistently occurred when chlorophyll a was available at only 1 to 2 pg 1. ' Although clearance rates decreased exponentially, rates of filtrat~on and ~ngestion (mg h ') continued to grow as the total dry part~culczte mass (TPV) of available seston increased u p to about 1000 n~g l ' The proportlon of t~ltered matter ejected a s pseudofaeces before lngestlon remained at a constant fractlon xvh~ch averaged 0 87 at seston concentrations abovcl about 40 mg TP>l l Only above about 1000 m g TPM I-' dld a decline In f~ltration rate suggest any physical ovcrload~ng of feedlng mechanisms Within particles filtered on the cten~dla significant net ennchment of up to 0 23 * 0 10 (mean + 2 SE) tlmes the organic fractlon in available seston stemmed pnnlanly from the preferential retention of particles that were nch in chlorophyll a The efficiency of such retention vaned in quadratic relat~on with seston co~npos~tlon b e~n g maximal at an optimal concentrat~on of chlorophyll a per unit total organic matter Within ingested particles, f u r t h e~ net enrichment of up to as much as 1 47 + 0 26 times the organic iractlon within filtered matter stemmed from the differential pre-ingestive relectlon of particles w~t h i n pseudofaeces Efficlencies of both these selective mechanisms increased with filtrat~on rates The combined effect was that the organic content of ingested matter (OCI) was enhanced at faster filtration rates, to as much as 7 times the organic content of available seston (OCS). Associated benefits were a m p u l e d by a positlve hyperbolic relation that was confirmed between OCJ and the net absorption efficiency from Ingested organlcs Flndlngs showed that tissue wasting occurred in P canaliculus of l g dry soft tissue feedlng upon seston that contained less than 0.86 + 0.19 pg chlorophyll a l . Maximal net organic absorption rate indicated potcntial growth of 6 S":, d ' g ' dry soft tissue, assuciated with the peak in clearance rate. A high capacity for f~ltrat~on and the ability to adjust clear,ince ratr cnabled P canaliculus to optimise particle selection and absorption efficiencies at levels that maintained organic absorption rate independent of the reduction in OCS as TPM increased to at least 1000 mg I ' Collective findings help to explain why P canaliculus~s so well-suited for cultivation, establish that reduced mussel growth within the New Zealand GreenshellT\' industry from 1996 to 1998 was due at least in part to food limitation, and suggest a potential for significant expansion of ...

Y l i "), O 'LO02 I]\ Tlie Xf)cological Societ~ of .-\mcrica. L.a~vrcncc. liS 6GO4+88Yi

SUMMARY The resistance and resilience of littoral zone communities to sedimentation will depend both on the extent to which sediment deposition affects productivity, and on interactions within the communities. A series of hypotheses were set up and tested to examine interactions and feedback mechanisms among deposited sediments, periphyton, macrophytes and grazers in a large oligotrophic lake subject to fluctuating sediment loadings. Although sediments incorporated into periphyton reduced light availability to macrophytes, periphytic algae were generally the dominant light absorbing component under natural conditions. When grazers were absent, both sediments incorporated in the periphyton and periphytic algal densities increased, and both were then important in reducing light available to macrophytes. Grazing rate and assimilation efficiency for the dominant grazer, the prosobranch gastropod Potamopyrgus antipodarum, increased with increasing sediment content under natural lake conditions to reach a maximum at 10 mg sediment cm−2. An increase in sediment incorporation into periphyton films resulted in an increased grazing rate and hence grooming of sediments from macrophytes. Grazing invertebrates can play a major role in maintenance of littoral communities by continuously grooming macrophyte hosts of periphytic algae and settled sediments.

No abstract

The macroinvertebrate community was surveyed at eight littoral zone sites in Lake Coleridge, New Zealand. Sites covered a range of conditions including exposure to prevailing winds, shore-line topography, and proximity to river inputs-which are a major source of suspended sediment to the lake. The littoral zone contained an abundant and diverse macroinvertebrate fauna (47 taxa). Three communities were identified: a shallowwater community inhabiting the wave-break zone, dominated by Trichoptera and chironomid larvae; a middle-zone community associated with macrophyte beds, with high macroinvertebrate abundance and dominated by gastropods and oligochaetes; and a community in fine sediments below the macrophytes dominated by oligochaetes. The depth distribution of these communities was related to substrate, macrophyte abundance, and shore-line profile. Superimposed on this pattern were the effects of hillside slope and exposure to prevailing north-west winds. Greatest abundance was associated with macrophytes on stable shores at 4-7.5 m depth, i.e., below the effective wavebase depth. Sediment deposition had only a localised direct effect close to where river water enters the lake. Indirect effects were mediated through the effects of suspended solids on water clarity, bottom limits, and production of macrophytes and periphyton. Estimates of the proportion of littoral macroinvertebrate biomass to whole-lake zooplankton biomass was 10:1 and for production was 1.3:1, indicating the potential importance of the littoral zone to whole-lake production.