Relative effects of elevated grazing pressure and competition from a red algal turf on two post-settlement stages of Fucusevanescens C. Ag.

In Nova Scotia, Canada, the lower shore of wave-exposed and semi-sheltered portions of the coast is dominated by a dense turf of the red alga Chondrus crispus C. Ag. The mid shore is dominated by three species of rockweed belonging to the genus Fucus, which are also physiologically competent in the low zone, but may be inhibited by pre-emptive competitive exclusion by red algal turf and/or increased grazing pressure from abundant mesoherbivores. A grazer assay showed that herbivore pressure is >50% higher in the Chondrus zone than in the rockweed zone. Fucus can colonize the low shore only through microrecruits which develop from planktonic zygotes. A main objective of this study was to test the hypothesis that early post-settlement stages represent a critical phase for successful recruitment. We, therefore, tested the relative effects of competitive pressure from Chondrus and grazing on small (2 mm) and larger (6 cm) Fucus evanescens plants in a three-way factorial experiment. Red algal turf and grazers independently depressed F. evanescens growth, but only a combination of factors resulted in a significant weight loss in the rockweeds. The effect size of grazing was more than double that of competition from the turf. Small and larger individuals of F. evanescens were equally affected by grazing and competition. The results showed that colonization by rockweeds may only occur where there are gaps in the Chondrus canopy. We hypothesized that grazer effects may decrease with increasing gap size. However, a controlled experiment showed no differences in Fucus recruitment among the range of naturally occurring gap sizes. That is, larger gaps do not promote rockweed recruitment. In concordance with this observation, we found that grazer abundances were similar among gap sizes. Chondrus was shown to be highly resistant to intense physical and herbivore-induced disturbance. This combination ensures the persistence of a dense canopy which largely excludes rockweed recruitment from zygotes. Competitive dominance of Chondrus on the low shore is not obviously related to individual growth rates or canopy height. A positive correlation between growth rate and competitive ability may be expressed only in morphologically similar species which are likely to compete as germlings.     

菹草提取物总黄酮、总酚酸和抗氧化活性含量研究

研究菹草提取物的有效成分,测定其总黄酮含量及总酚酸含量,并确定它的乙酸乙酯和石油醚部位的抗氧化活性成分。采用紫外可见分光光度法,以芦丁和没食子酸作为控制材料,测定菹草的石油醚部位及乙酸乙酯部位中总黄酮和总酚酸含量;采用清除DPPH自由基能力测定法、测定总还原能力铁氰化钾法和水杨酸捕捉羟基自由基法来对菹草各组分提取物的抗氧化能力进行研究。菹草中有一定量的黄酮类化合物和酚酸类化合物存在,不同溶剂提取的部位所含有的总黄酮和总酚酸含量是有差异的,石油醚部位的总黄酮含量要高于乙酸乙酯部位的总黄酮含量,并且总酚酸的含量亦是如此;石油醚部位和乙酸乙酯部位的提取物具有清除DPPH、羟基自由基和还原Fe^3+的能力,各部位清除DPPH、羟基自由基的能力和还原Fe^3+的能力随着样品浓度的增大而增大,且乙酸乙酯部位的测定结果均低于石油醚部位。     

THE ZOOBENTHOS OF THE TOUW RIVER FLOODPLAIN

SUMMARY

An eighteen month study (January 1979 - June 1980) of the sediment, emergent and submerged plant-associated benthic macro-invertebrates of the western sector of the Touw River Flood-plain is reported for four study sites: Wilderness Lagoon, the Touw River and East and West Serpentine. Total invertebrate standing stocks on emergent vegetation were negligible. Those associated with submerged plant communities (essentially Potamogeton pectinatus L.), were generally very much greater than sediment-associated invertebrate standing stocks. The following mean biomasses were recorded for the study period: (a) sediments - Wilderness Lagoon, 19,25; Touw River, 13,06; West Serpentine, 4,37; East Serpentine, 4,68 g dry mass m^?2; (b) - Potamogeton-associated fauna - Touw River, 128,9; West Serpentine, 91; East Serpentine, 35,78 g dry mass m^?2. Of the 37 taxa recorded for Wilderness Lagoon, 33 for Touw River and 31 each for West and East Serpentine sites, the most consistently dominant species were the amphipods, Melita zeylanica Stebbing, Corophium triaenonyx Stebbing, and Grandidierella lignorum Barnard, the tanaid, Apseudes digitalis Brown, and the bivalve mollusc, Musculus virgiliae Barnard. However, suspension feeding Musculus was by far the most important taxon.

In addition to normal winter die-back of Potamogeton, submerged plant cutting activities in the Serpentine lead to an extended period of plant senescence, a prolonged recovery period at the West Serpentine station (between 4–5 months, compared with un-cut Touw River - approximately 2 months), almost complete failure of the plant at East Serpentine, and associated reductions or shifts in the invertebrate community standing stocks, composition and structure. Sediment-associated invertebrate standing stocks at both West and East Serpentine stations increased after Potamogeton harvesting, both immediately and in the long-term. However, these sediment fauna increases nowhere near compensated for the reductions of plant-associated faunal standing stocks due to cutting activities. The significance of these findings is discussed in terms of future management of the area, and comparisons with the situation in Swartvlei, where natural Potamogeton reduction occurred, and with other estuarine and coastal systems, are made.     

Lack of detectable isozyme variability in British populations of Potamogeton epihydrus (Potamogetonaceae)

Outside North America Potamogeton epihydrus Raf. is confined to two areas of Britain. An investigation of British material from apparently native sites in the Outer Hebrides and from a canal in northern England (where it is believed to be introduced) revealed only a single isozyme genotype. Ten putative loci were resolved. The IDH phenotype was uniform and three-banded, consistent with heterozygosity (possibly caused by a duplicated locus or preserved by clonal growth). All other enzyme loci were homozygous.