Stephen G. Nelson

Salt tolerance and osmotic adjustment of Spartina alterniflora (Poaceae) and the invasive M haplotype of Phragmites australis (Poaceae) along a salinity gradient.

An invasive variety of Phragmites australis (Poaceae, common reed), the M haplotype, has been implicated in the spread of this species into North American salt marshes that are normally dominated by the salt marsh grass Spartina alterniflora (Poaceae, smooth cordgrass). In some European marshes, on the other hand, Spartina spp. derived from S. alterniflora have spread into brackish P. australis marshes. In both cases, the non-native grass is thought to degrade the habitat value of the marsh for wildlife, and it is important to understand the physiological processes that lead to these species replacements. We compared the growth, salt tolerance, and osmotic adjustment of M haplotype P. australis and S. alterniflora along a salinity gradient in greenhouse experiments. Spartina alterniflora produced new biomass up to 0.6 M NaCl, whereas P. australis did not grow well above 0.2 M NaCl. The greater salt tolerance of S. alterniflora compared with P. australis was due to its ability to use Na(+) for osmotic adjustment in the shoots. On the other hand, at low salinities P. australis produced more shoots per gram of rhizome tissue than did S. alterniflora. This study illustrates how ecophysiological differences can shift the competitive advantage from one species to another along a stress gradient. Phragmites australis is spreading into North American coastal marshes that are experiencing reduced salinities, while Spartina spp. are spreading into northern European brackish marshes that are experiencing increased salinities as land use patterns change on the two continents.

Cultivation of Gracilaria parvispora (Rhodophyta) in shrimp-farm effluent ditches and floating cages in Hawaii : a two-phase polyculture system

Abstract A culture system for the commercial production of the seaweed Gracilaria parvispora using shrimp-farm effluents for fertilization and floating cage-culture for grow-out has been developed on Molokai, HI. This two-phase system produces high-quality products for direct human consumption. The mean relative growth rates (RGRs) of effluent-enriched thalli in the cage system ranged from 8.8% to 10.4% day−1, a significant increase over the growth (4.6% day−1) of thalli fertilized with inorganic fertilizer. Thalli were also grown directly in the effluent ditch, where mean growth rates of 4.7% day−1 were obtained, less than in cage-culture. In the cage-culture system, thallus nitrogen content declined without fertilization. Effluent-enriched thalli grown in the cages steadily declined in nitrogen content, to about 1%, and their C:N ratios increased to between 20 and 30. However, when nitrogen-depleted thalli were transferred to the effluent ditch for enrichment, N content rapidly increased over 5 days to approximately 3%, with a C:N ratio near 10. Benefits of this two-phase polyculture system include enhanced growth of G. parvispora and the use of effluent from commercial shrimp farms as a resource.

Effects of fertilization treatment and stocking density on the growth and production of the economic seaweed Gracilaria parvispora (Rhodophyta) in cage culture at Molokai, Hawaii

Abstract The edible red seaweed, Gracilaria parvispora Abbott, was pulse-fertilized in tanks containing fish-culture water or chemical fertilizer, then cultured in floating cages in a low-nutrient, ocean lagoon in Molokai, HI. Small, daily additions of ammonium sulfate and ammonium diphosphate were the only additions needed to stimulate growth. Fish-culture water was as effective as chemical fertilizer in supporting growth. Thalli fertilized for 7 days in tanks contained 2.5–5% nitrogen in tissues by the end of the treatment period; upon transfer to low-nutrient water, nitrogen content decreased to 1% as the nitrogen was mobilized to support growth. Thalli grew rapidly over the first 14 days after transfer from fertilizer tanks to the ocean, achieving relative growth rates of 8–10% day −1 and producing 39–57 g dry wt. m −2 day −1 . However, by 21 days after transfer, growth ceased due to depletion of stored nutrients. The optimal stocking density was 2 kg m −3 based on growth rates. Nearly all net growth occurred in the cages rather than in the fertilizer tanks, which serve only to introduce nitrogen into the thalli. The yields obtained here are four times higher than achieved previously with this species and are comparable to high-yielding, intensive tank cultures.

Effects of a Dam and Reservoir on the Distributions and Densities of Macrofauna in Tropical Streams of Guam (Mariana Islands)

ABSTRACT Streams on the island of Guam were surveyed to compare species composition and densities of the most common species among seven rivers, of which three were above a dam and reservoir and three were unregulated. Rivers above the reservoir were also compared to the outflowing river. Species found in all of the rivers included the indigenous gobies Awous guamensis and Stiphodon elegans and two introduced cichlid fishes, Oreochromis mossambicus and Tilapia zillii. The gobies have modified pectoral fins that allow them to climb waterfalls and to traverse the spillway and enter the reservoir from downstream. The cichlids, however, are not morphologically adapted for climbing and were introduced to the reservoir; they eventually moved into the rivers. The freshwater shrimp Macrobrachium lar, smaller atyid shrimp of the genera Atya and Caridina, and the gastropod Thiara granifera were also found above the reservoir, in the outflowing river, and in the unregulated rivers. However, the flagtail Kuhlia rupes...

Growth and Distribution of the Macroalgae Gracilaria salicornia and G. parvispora (Rhodophyta) Established from Aquaculture Introductions at Moloka'i, Hawai'i

Abstract: Gracilaria salicornia and G. parvispora were introduced to the south reef of Moloka‘i, Hawai‘i, in the past 15–20 yr for aquaculture development. Both species have naturalized on the reef. Gracilaria salicornia is now considered an invasive species on O‘ahu due to its tendency to grow in dense beds that produce undesirable windrows of thalli on the beach. There is also concern that it reduces biodiversity and degrades habitats of reefs. We surveyed the south coast of Moloka‘i, where both species were introduced, and measured biomass density, growth rates, and thallus nutrient contents of G. salicornia in established beds. Both species are found in the silt-laden, nearshore zone of the reef within 50 m of shore. Gracilaria salicornia grows in dense beds containing 475 g dry weight m-2 of biomass, but growth rates are low, 0.03%–1.28% day-1. Tissue nitrogen levels are low, suggesting that these populations are nitrogen limited. Nevertheless, populations of G. salicornia persist and grow slowly on the reef, whereas those of G. parvsipora are only found in areas of local nitrogen enrichment from anthropogenic sources. Currently, G. salicornia does not appear to be negatively affecting the reef ecology on Moloka‘i, because it is confined to the disturbed, nearshore zone. However, its ability to grow slowly and persist under low-nitrogen conditions allows it to form dense beds and suggests that it will eventually spread farther along the coast.