G.Z. Chen

Effect of wastewater discharge on nutrient contamination of mangrove soils and plants

The ecological impact of sewage discharges to a mangrove wetland in Futian National Nature Reserve, the People’s Republic of China was assessed by comparing the plant community, plant growth and nutrient status of soils and vegetation of a site treated with settled municipal wastewater (Site A) with those of a control adjacent site (Site B) which did not receive sewage. During the one year study, the total and available N and P, and organic carbon concentrations of mangrove soils in Site A did not significantly differ from those of Site B. In both sites, the soil organic C, total N, NH 4 + -N and total P content exhibited a descending trend from landward to seaward regions, with the lowest measurements obtained from the most foreshore location. Seasonal variation in N content of soil samples was more obvious than any difference between wastewater treated and the control sites. The soil N content was lower in spring and summer. This was attributed to the higher temperature in these seasons which facilitated degradation of organic matter and absorption of nitrogen by the plants for growth. No significant difference in plant community structure, plant growth (in terms of tree height and diameter) and biomass was found between Sites A and B. Leaf samples of the two dominant plant species, Kandelia candel and Aegiceras corniculatum collected from Site A had comparable content of organic carbon, N, P and K to those of Site B. These preliminary results indicated that the discharge of a total volume of 2600 m3 municipal wastewater to an area of 1800 m2 mangrove plants over the period of a year did not produce any apparent impact on growth of the plants. The soils and plant leaves of Site A were not contaminated, in terms of nutrient content, by the discharged sewage.

The role of radial oxygen loss and root anatomy on zinc uptake and tolerance in mangrove seedlings

Root anatomy, radial oxygen loss (ROL) and zinc (Zn) uptake and tolerance in mangrove plants were investigated using seedlings of Aegiceras corniculatum, Bruguiera gymnorrhiza and Rhizophora stylosa. The results revealed that B. gymnorrhiza, which possessed the tightest barrier in ROL spatial patterns among the three species studied, took up the least Zn and showed the highest Zn tolerance. Furthermore, zinc significantly decreased the ROL of all three plants by inhibition of root permeability, which included an obvious thickening of outer cortex and significant increases of lignification in cell walls. The results of SEM X-ray microanalysis further confirmed that such an inducible, low permeability of roots was likely an adaptive strategy to metal stress by direct prevention of excessive Zn entering into the root. The present study proposes new evidence of structural adaptive strategy on metal tolerance by mangrove seedlings.

Effect of synthetic wastewater on young Kandelia candel plants growing under greenhouse conditions

A greenhouse experiment was performed to evaluate the effects of synthetic wastewater in three different strengths, NW, MW and CW, on the growth of the one-year old Kandelia candel (L.) Druce plants. NW had the characteristics and strength similar to natural municipal wastewater while MW and CW contained five and ten times of the nutrients and heavy metals in NW, respectively. Artificial seawater was used as the control. During one year wastewater treatment experiment, Kandelia were found to withstand wastewater of high strength and toxic symptoms were not detected in all plants. Synthetic wastewater with strength similar to the natural municipal sewage (NW) stimulated plant growth. The plants treated with NW had significantly higher aerial and root biomass, taller stem than those found in the CW, MW and the control. Maximum growth, in terms of both stem height and total biomass, of all plants occurred in summer months, from June to September. With respect to the physiological and biochemical activities, CW and MW treated plants had significantly lower levels of chlorophyll a, total chlorophyll and catalase activity than those found in NW and control groups. In contrast, the proline content of plants treated with wastewater was similar to that of the control. These results suggest that normal wastewater (NW), attributed to its nutrients and trace elements, enhanced plant growth. The medium (MW) and concentrated wastewater (CW) supported similar amount of plant growth as the control but the physiological and biochemical parameters indicate that these treated plants might have been exposed to some kind of stress, probably due to the excess heavy metals present in MW and CW.

Benthic macrofauna of the mangrove swamp treated with municipal wastewater

Benthic macrofauna and possible effects on community structureofdischarging municipal wastewater to Futian National MangroveReserve, the People‘s Republic of China, were investigated.Eighty-four species of ground-dwelling fauna belonging to 7Phyla,9 Classes, 46 Families and 65 Genera were identified. Abundantfamilies were Assimineidae, Melanidae, Potamididae,Ocypodidae,Grapsidae and Periophthalmidae. Between April 1991 to January1993,no obvious seasonal fluctuation in species compositionoccurred.Total biomass also remained unchanged. However, total densityshowed peaks in January and July (both in section A and B),probably due to the molluscs.A selected site of the mangroves was treated with municipalwastewater (60 m3 per week for 16 consecutive months). Incomparison to a control site, the effect of wastewaterdischarge ontotal mean biomass and density (particularly of molluscs andcrustaceans) appeared non-significant P?0.05). Thecommunity structure remained unchanged although some saprobicspecies emerged and total biomass in the area close to thewastewater front increased slightly. Multivariate analysisindicated that there probably was an initial inhibition periodanda subsequent recovery period for benthic fauna in response tothedischarged wastewater in the mangrove swamp.The study confirms that the benthic macrofauna withinmangroves islow in species diversity and high in population counts incomparison with a frontal mudflat. Longitudinal zonation ofthefauna from landward to seaward will also bediscussed.

Community Structure and Standing Crop Biomass of a Mangrove Forest in Futian Nature Reserve, Shenzhen, China

The community structure and standing crop biomass of a mangrove forest in Futian Nature Reserve, Shenzhen, the People’s Republic of China was studied. This mangrove, located towards the northern latitudinal limit of the mangrove development (22°N), had relatively simple structure and low diversity (the Shannon-Wiener index was 0.78). The three dominant species, namely Aegiceras corniculatum, Kandelia candel and Avicennia marina, possess importance values of 72, 19 and 9%, respectively. The average height of the mangrove community was 4.5 m with no vertical stratification. The values of tree density of A. corniculatum and A. marina were found to be 5290 and 260 ha−1, respectively. The biomass of both A. corniculatum and K. candel was best estimated from regression equations using a combination of height and diameter at breast height as the independent variables. For Avicennia marina, there was no simple correlation between biomass and height or diameter. The regression models suggested by previous workers did not give satisfactory estimation of biomass of A. marina in this mangrove forest. The total biomass of this mangrove forest was 12.1 kg m−2, with 73% of such production contributed by A. corniculatum and 8% by A. marina. Because of its small percentage, the inaccuracy in estimating biomass of A. marina did not affect the overall determination of biomass of the whole community. Average above-ground biomass was 8.7 kg m−2 (72% of the total biomass) and the major components were aerial woody tissues, stems and branches. The rootshoot ratio of this plant community was 0.4:1.