The quest to discover simple, sensitive and global bioindicators of nutrient enrichment and ecological health continues. The present study is the first to address this quest in Hervey Bay by investigating links between key physico-chemical water quality parameters and two diatom forms. Free floating in the water column as phytoplankton, and attached to seagrass leaves and artificial substrates as microalgal epiphytes, diatoms are assessed for their abundance and diversity along an environmental gradient. Relationships between Secchi depth (Zsd), light attenuation coefficient (Kz) and minimum seagrass compensation depth (Zc) are established for the recovery, growth and survival of aquatic plants following disturbance, with equations derived specifically for this region to allow conversions from Zsd to Kz. These relationships allow predictions to be made and provide a management tool to meet water quality targets and combat elevated nutrient and sediment loads that result from anthropogenic activity and affect all marine life, from microalgae to marine mammals. Environmental links, identified as drivers of change to biodiversity, focus research effort and provide predictive management tools. Consequently, catchment and coastal activities such as dredging, construction and canal development, with the potential to increase nutrient and sediment loads, can be managed and scheduled at times least likely to adversely impact near shore ecosystems and therefore improve the balance between growth and sustainability. Seasonal and zone differences were significant for many of the water quality parameters monitored in Hervey Bay during 1994. Water clarity measures consistently showed significant differences between near and offshore zones (Secchi depth; P<0.01, turbidity; P<0.05 and TSS; P<0.01) as well as oxygen (P<0.01), soluble reactive phosphate (SRP, P<0.05), oxides of nitrogen (NOx, P<0.01), chlorophyll a (Chl a, P<0.01) and pH (P<0.01). Some default trigger values for water quality targets listed by ANZECC/ARMCANZ (2000) for concentrations of key indicators, a threshold for risk of adverse biological effects, were exceeded in dry and flood periods during the 1993 to 1995 survey. The community structure of phytoplankton and seagrass epiphytes were examined by monitoring changes to and 35 seagrass epiphyte species coincident with changes in some key water quality parameters along a water quality gradient. Chl a, Secchi depth (Zsd), soluble reactive phosphate (SRP) and pH were found to be the best subset of water quality parameters to maximise the rank correlation with phytoplankton communities whereas NOx and temperature maximised the rank correlation with the Halophila ovalis seagrass epiphyte communities. These parameters statistically link key water quality parameters to changes in phytoplankton and seagrass epiphyte density and assemblage structure and are supported as significant drivers of change in biodiversity research. Evidence for nitrogen limitation was found in the post flood surveys for phytoplankton and the growth and assemblage structure for seagrass systems and their epiphytes. Seasonal and zone differences in phytoplankton assemblage structure were most apparent with some site influence detected. Cylindrotheca closterium dominated the phytoplankton assemblage structure at the near shore zone while Thalassionema nitzschioides and Chaetoceros sp. differentiated the river from the Waste Water Treatment Plant (WWTP) creek site. Rhizolsolenia and Guinardia sp. were found in significantly higher concentrations at offshore sites (P < 0.01) and Thalassionema nitzschioides (P < 0.01), Thalassiosira sp. (P < 0.01) and Pseudonitzschia sp. (P < 0.05) were found in significantly higher abundance in the near shore zone A Trichodesmium bloom was examined and post flood changes to the phytoplankton assemblage structure associated with increased nutrient loads, reduced water clarity (Secchi depth, TSS and turbidity) and changes to pH and salinity were assessed. The results of analyses of variance provided support to multivariate statistical analyses to identify phytoplankton as a useful and sensitive bioindicator of environmental change. Post flood phytoplankton cell density increased and species diversity rose from 10 to 38 species at the Mary River mouth however, the changes to cell density and assemblage structure were not reflected in Chl a concentrations. The Mary River mouth experienced growth of mainly small phytoplankton species (< 20 µm) while Pulgul Creek, a source of WWTP effluent, experienced a disproportional increase in the larger phytoplankton species (>20µm). The increase in phytoplankton cell density at offshore sites occurred for both large and small species. Phytoplankton species dominance changed at each site during the postflood period. C. closterium remained the dominant species but increased from 34 to 648 cells/mL at Pulgul Creek however, C. similis, the second most dominant species was replaced by Pseudonitszchia sp., whose cell density rose from 2.0 to 320 cells/mL. A recent study also found that Cylindrotheca closterium, Skeletonema costatum, and Cyclotella choctawhatcheeana had strong positive relationships with coastal nutrients and suggested these species be used as potential reliable indicators of eutrophication (Toming and Jaanus, 2007). Some Pseudonitszchia sp. and dinoflagellate species have previously been noted as toxic and Chaetoceros sp. are associated with gill damage and fish kills. Rhizosolenia sp. and the Guinardia species G. flaccida and G. striatula, were indicators for the offshore relatively pristine sites, which may conversely, determine these species as indicators of environmental health in this waterway. Specificity of epiphyte attachment to particular seagrass species were detected and microalgal epiphyte assemblages on Halophila ovalis were identified as useful bioindicators of environmental gradients as this seagrass species is located at intertidal, shallow and deep water sites for a substantial part of the year. H. ovalis also recorded the highest average annual epiphyte loads (3873 ± 1882 cells/mm2) with the diatoms Diatoma vulgare and Cocconeis scutellum representing 19.6% and 17.2% of the total epiphyte cover. Highest cell density was recorded in autumn, followed by summer and winter: spring recorded the lowest epiphyte cover. Micro-algal epiphyte load was assessed as dry weight (g/m2), Chl a (µg/L) and cell density (cells/mm2) on artificial seagrass deployed along a water quality gradient. Equations were derived to describe percentage light transmission as a function of each measure of epiphyte load which allows transformation of data from one unit of expression to another and hence, allow comparison of past, current and future studies. Water column light attenuation was evaluated along a water quality gradient and linked to Secchi depths (Zsd) to derive equations that describe this relationship where for Hervey Bay, when Zsd < 4.26m, Kz = -1.2 ln Zsd + 1.74 and when Zsd ≥ 4.26m, Kz=1.37/Zsd. Species succession, diversity and seagrass growth were monitored for the first time during a recovery phase and limits for the seagrass compensation depth (Zc), were established on a seasonal basis for five seagrass species at four locations within intertidal, shallow and deep water zones in Hervey Bay and the Great Sandy Straits. Conceptual models for each site were constructed to describe habitat characteristics and include nutrient concentrations for SRP, NOx and ammonia (NH4+), Chl a, phytoplankton density, epiphyte density and light requirements measured as light attenuation, total suspended sediment, turbidity and Secchi depth at each location. The study provides a descriptive model for light attenuation and establishes (1) an annual and seasonal baseline water quality data set that characterises the waterways of Hervey Bay and the Great Sandy Straits (2) identification of phytoplankton species in Hervey Bay and their response to post flood changes in water quality (3) phytoplankton density and diversity along an environmental gradient with links established to Chl a, Secchi depth, SRP and pH (4) seagrass epiphyte assemblage structure along an environmental gradient with links established to NOx and temperature (5) conversion factors for epiphyte load expressed as dry weight, Chl a and cell density (6) functions to convert Secchi depth to light attenuation coeffiecients (Kz) (7) seagrass species succession at intertidal, shallow and deep water sites during a recovery growth phase in Hervey Bay (8) seagrass compensation depths (Zc) for five seagrass species at four locations and (9) habitat characterisation for seagrass recovery in Hervey Bay. Identification catalogues for phytoplankton and seagrass epiphytes were prepared with light and electron micrographs to assist future identification studies of diatoms in this region and for other similar biogeographical areas. The management of water quality to reach specific targets requires the capacity to predict seagrass compensation depth (Zc) as a function of water quality. Seasonal and annual light attenuation measurements are derived as a function of water quality at intertidal, shallow and deep water habitats during the colonisation of Zostera Capricornii, Halodule uninervis, Halophila ovalis, Halophila spinulosa and Halophila decipiens.
| Identifer | oai:union.ndltd.org:ADTP/279233 |
| Creators | Deborah Milham Scott |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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