Tracing the fate of N-15-enriched feed in an intensive shrimp system

The fate of N-15-nitrogen-enriched formulated feed fed to shrimp was traced through the food web in shallow, outdoor tank systems (1000 1) stocked with shrimp. Triplicate tanks containing shrimp water with and without sediment were used to identify the role of the natural biota in the water column and sediment in processing dietary nitrogen (N). A preliminary experiment demonstrated that N-15-nitrogen-enriched feed products could be detected in the food web. Based on this, a 15-day experiment was conducted. The ammonium (NH4+) pool in the water column became rapidly enriched (within one day) with N-15-nitrogen after shrimp were fed N-15-enriched feed. By day 15, 6% of the added N-15-nitrogen was in this fraction in the 'sediment' tanks compared with 0.4% in the 'no sediment' tanks. The particulate fraction in the water column, principally autotrophic nanoflagellates, accounted for 4-5% of the N-15-nitrogen fed to shrimp after one day. This increased to 16% in the 'no sediment' treatment, and decreased to 2% in the 'sediment' treatment by day 15. It appears that dietary N was more accessible to the phytoplankton community in the absence of sediment. The difference is possibly because a proportion of the dietary N was buried in the sediment in the 'sediment' treatment, making it unavailable to the phytoplankton. Alternatively, the dietary N was retained in the NH4+ pool in the water column since phytoplankton growth, and hence, N utilization was lower in the 'sediment' treatment. The lower growth of phytoplankton in the 'sediment' treatment appeared to be related to higher turbidity, and hence, lower light availability for growth. The percentage N-15-nitrogen detected in the sediment was only 6% despite the high capacity for sedimentation of the large biomass of plankton detritus and shrimp waste. This suggests rapid remineralization of organic waste by the microbial community in the sediment resulting in diffusion of inorganic N sources into the water column. It is likely that most of the dietary N will ultimately be removed from the tank system by water discharges. Our study showed that N-15-nitrogen derived from aquaculture feed can be processed by the microbial community in outdoor aquaculture systems and provides a method for determining the effect of dietary N on ecosystems. However, a significant amount of the dietary N was not retained by the natural biota and is likely to be present in the soluble organic fraction. (C) 2002 Elsevier Science B.V. All rights reserved

Potential for longevity of novel genetically modified herbicide-tolerant traits in the Irish landscape

With the renewed interest in GM crop technology in Ireland, some concern has been raised in relation to the potential impact on biodiversity in the Irish agri-environment. This concern can focus on the potential for a transgenic trait to cross to wild relatives. A novel trait will be judged to have persisted in a wild population via the successful production of seeds, such that these seeds are viable and result in the establishment of a self-sustaining population. In the case of a herbicide tolerant (HT) trait, feral and volunteer populations can only remain viable if managed with applications of the herbicide that the trait is designed to resist. This surviving population of HT plants would then need to compete successfully with other wild plants in order to prevail in the landscape and persist over time. There are few agricultural crops that can manage this combination, but as oilseed rape plants are often noted along roadsides and hedgerows in Ireland, it is correct to assume that this crop has the ability to be a successful feral survivor. This paper presents the results of a thought experiment, derived exclusively using the academic literature, on the issue of longevity. This is done by taking four hypothetical case scenarios and examining the potential for a combination of events to take place for oilseed rape (Brassica napus), selected here because it has a high potential for 'escaping' via pollen- and/or seed-mediated gene flow. A lack of quantitative data on Irish farmland biodiversity hinders solid conclusions, but when management pressure is eased biodiversity stress is lessened.EPA STRIVETeagascAuthor has checked copyrightRB 17/04/1