Plant Conservation outside reserves in the lower North Island
- Author
- Sawyer, J.W.D.
- Year
- 1997
- Journal / Source
- Hale, P; Lamb, D (Eds), Conservation Outside Nature Reserves, The University of Queensland, Queensland
- Summary
- Agricultural and urban development can increase run-off and lead to excessive nutrient loadings in fragile coastal environments that are nursery grounds for a diverse array of coastal and estuarine species, as well as other resident organisms. This project investigates the development of bioindicators to strengthen the ability of managers to detect and quantify changes in anthropogenic nitrogen inputs to coastal and estuarine ecosystems by comparing six study sites with different levels of development ranging from pristine through to fully urban. The results show a strong positive relationship between the percent agricultural land in surrounding catchments and total nitrogen (TN) loading to nearshore environments. Stable nitrogen isotope ratios (?15N) of primary producers increased with increasing water column dissolved inorganic nitrogen (DIN) concentration and TN loading. The ?15N value of macroalgae (Ulva spp.) was clearly related to wastewater nitrogen loads in coastal environments, thus demonstrating that it is a good indicator of land-derived nutrients around urban watersheds. This relationship was most significant during the algal growth period (spring-summer), suggesting that sampling macroalgae to detect regional differences in isotopic values should be conducted in summer. By contrast, isotope ratios of primary consumers (filter-feeding bivalves) showed no clear relationship to different terrestrial nutrient sources among estuaries. However, there was a clear spatial gradient within individual estuaries, which was decoupled from the isotopic gradients of primary producers. These results hint at differences in dissolved and particulate nitrogen source pools, and highlight the importance of using complementary components of food webs and high spatial replication to show linkages between watershed land use and chemical markers in biota. The effects of nutrient enrichment were transmitted up the food web, with growth of secondary consumers, Notolabrus celidotus (spotties) and Grahamina nigripenne (estuarine triplefins) generally enhanced in nutrientenriched coastal areas. Benthic prey dominated the diets of these fish species, with amphipods and brachyurans being the most important prey items for triplefins and spotties, respectively. However, there were site-specific differences in prey importance and diet diversity. Both triplefins and spotties consumed considerably more diverse prey items at pristine than nutrient-enriched coastal areas. Food web models based on stomach content analyses and dual isotope ratios suggest that there are shifts in the relative importance of the different organic matter sources supporting food structure among the different coastal ecosystems due to nutrient enhancement from land-based activities.
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