Habitat point records from 1989 FSCRC Blackshaw Bank (Solway) cockle dredging study

The effects of intensive hydraulic cockle dredging during one tidal cycle on the sediment and infaunal community compostion of two areas of sediment flat were studied during the period June 1989 - January 1990. The 2 areas selected for the study were on the mid-shore of Lavan Sands, N Wales and on the mid-shore of Blackshaw Flats, Solway Firth. The infaunal community of the Lavan Sands (LS) sites were dominated by the cockle C. edule, the baltic tellin shell M. balthica, the lugworm A. marina and the amphipod C. arenarium, with a variety of small polychaetes and other infaunal species. The infaunal community of Blackshaw Flats (BF) sites was less diverse, and dominated by cockles, baltic tellin shells and the small snail H. ulvae, with a variety of other small infaunal species. 6 sites were established in each area, 3 to act as controls for the 3 experimental sites which were dredged during one tidal cycle during June (LS) or July (BF). The sites were sampled for infaunal and sedimentological analyses on the day before dredging and then one day, one week, 3 months and 6 months after dredging. The data were analysed with the aid of a variety of statistical and graphical techniques. Prior to dredging, there was only a smally variability between the sediment characteristics of the 6 sites, at both LS and BF. However, the abundances of many infaunal species varied significantly between sites and between the control and experimental areas, particularly at LS. This is considered to be due extreme patchiness in the distribution patterns of the infauna, rather than to differences in the environmental characteristics of the sites. Comparison of the experimental sites the day before and the day after dredging found very few differences in sediment characteristics, although there was a small but significant increase in the mean grain size of the sediment at BF. There was no significant difference in the precentage mud content. Immediate effects on the infauna at LS were considerable, with significant reductions in the abundances of most species. The total number of recorded taxa dropped from 27 the day before to 20 the day after. The mean abundance of all individuals was reduced from 7,000 to 3,300 per squ. m, while the precentage reduction in abundance of most individual species was around 50%. However, at BF the effects were less dramatic, with significant reductions of only one species H. ulvae, reduced from a mean abundance of 3.313 to 1.293 per squ. m. Analysis of seasonal trends at all sites over the 6 months after dredging found that the sediment characteristics showed very little variation at either LS or BF. The percentage organic matter of the LS sites did show a small but significant reduction from summer to winter, at both the control and experimental sites. Seasonal variations in the fauna, however, were significant for most species, and at LS there was a peak in the abundance of most species in the October samples. At BF the individual species showed very different trends from each other. Comparison of the control and experimental sites over the 6 month 'recovery period' found that the fauna appeared to have recovered from the efects of the dredging within 3 months, and few effects were visible 1 week after dredging. Although there were significant differences in some faunal parameters at all sampling times, these differences were probably due to natural variation.

在1989年6月至1990年1月期间，对两次潮周期内高强度水力蛤蜊疏浚对两处泥滩沉积物及底栖生物群落组成的影响进行了研究。所选取的两个研究区域分别位于威尔士北部的拉万沙洲中部海岸和索尔威湾的黑沙滩中部海岸。拉万沙洲（LS）地区的底栖生物群落以蛤蜊C. edule、波罗的海薄壳贝M. balthica、海蚯蚓A. marina和等足类C. arenarium为主，以及多种小型环节动物和其他底栖物种。黑沙滩（BF）地区的底栖生物群落多样性较低，主要由蛤蜊、波罗的海薄壳贝和小蜗牛H. ulvae组成，以及其他多种小型底栖物种。在每个区域建立了6个采样点，其中3个作为对照，用于与6月（LS）或7月（BF）期间进行疏浚的3个实验点进行对比。在疏浚前一天以及疏浚后的一天、一周、三个月和六个月对采样点进行了底栖生物和沉积物学分析。数据通过多种统计和图形技术进行分析。疏浚前，LS和BF地区的6个采样点的沉积物特征之间仅有微小差异。然而，许多底栖物种的丰度在采样点之间以及对照和实验区域之间存在显著差异，尤其在LS地区。这被认为是由于底栖生物分布模式的极端不均匀性，而非环境特征差异。对比疏浚前后的实验点，沉积物特征几乎没有差异，尽管在BF地区的沉积物平均粒度有轻微但显著的增加。泥质含量的百分比没有显著变化。对LS地区的底栖生物的即时影响显著，大多数物种的丰度显著减少。记录的物种总数从疏浚前的27种降至疏浚后的20种。所有个体的平均丰度从每平方米7,000个降至3,300个，而大多数物种个体丰度的百分比减少约为50%。然而，在BF地区的影响较小，仅一种物种H. ulvae的丰度显著减少，从平均每平方米3.313降至1.293。对疏浚后六个月内所有采样点的季节性趋势分析表明，LS和BF地区的沉积物特征几乎没有变化。LS地区的有机物含量百分比从夏季到冬季在对照和实验点均显示出微小但显著的减少。然而，大多数物种的动物群季节性变化显著，LS地区的物种丰度在10月样本中达到峰值。BF地区的物种个体表现出非常不同的趋势。对照和实验点在疏浚后六个月的'恢复期'比较发现，动物群似乎在三个月内从疏浚的影响中恢复过来，疏浚后一周内几乎看不到任何影响。尽管在所有采样时间点某些动物群参数存在显著差异，但这些差异可能归因于自然变异。