The ecology of a Zostera noltii bed ecosystem in the Solent

Abstract

There is a scarcity of seagrass bed studies along the British coast inspite of numerous works conducted in the subtropics and temperate regions. This study observed the biological and chemical aspects of seagrass, sediment pigment production and macrozoobenthos dynamics of Zostera noltii bed on the Isle of Wight during September 1997 to October 1999 from the dense and sparse sites. Seasonal patterns occurred in all studied seagrass biology aspects. Average values of shoot density (276-1020 stands-m"^), shoot height (13-23.2 cm), shoot leaf number (2.4-5.7) were maximum in the summer and autumn, whilst shoot biomass (2.1- 22.3 g DW-m"^) had the highest values in the autumn. Z noltii growth continued through April-October in each year. Reproduction was mainly through rhizomal growth, although flowering was recorded. Except for shoot leaf number per plant and shoot height, all variables were in higher values at the dense site. Of the chemical constituents measured, only protein did not vary. There were no clear differences in the contents of total carbohydrate, phenolics and protein of Z noltii between the two different sites. Bed pigment production varied according to the season and site, and was mainly derived from the microphytobenthos chl-a in the sediment. The dense site always had higher average concentartions of chl-a than the sparse site, both through spectrophotometer (3.25-14.24|ig-g"' Vs 2.76-10.20 pg-g'^) and HPLC (0.81-5.32 pg-g'^ Vs 0.66-4.24 pg-g'') analyses. Fucoxanthin values correlated with that of chl-a, indicating the main source of pigment was generated from diatoms. To a lesser extent, Z noltii detritus contributed to the bed pigment production. Other pigments resulting from HPLC measurements were chl-c7+c2, lutein/zeaxanthin,/-carotene, diatoxanthin and diadinoxanthin. Phaeopigment species, phaeophorbide a and phaeophytin a were also detected by HPLC, indicates the occurrences of macrozoobenthos grazing. The causes for higher values of chl-a and phaeopigment at the dense site were discussed. The macrozoobenthos variables measured varied seasonally and most of them varied temporally. All trophic levels in the food chain were represented. Of 124 species found, the polychaetes (49 species), amphipods (29 species) and gastropods (15 species) composed the three most dominant groups accordingly. Macrozoobenthic abundance, biomass, species number and species richness decreased from the dense to the sparse sites, and from the warmer months to the winter periods, also species number was higher in comparison with many other seagrass beds. The higher degrees of species similarity occurred among the samples from the same season and between the spring and autumn samples. The possible reasons for these were presented. This study emphasised the importance of seagrass bed in supporting the complexity of the food web within the system.

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