Deterioration of Sediment Quality in Seagrass Meadows (<Emphasis Type="Italic">Posidonia oceanica</Emphasis>) Invaded by Macroalgae (<Emphasis Type="Italic">Caulerpa</Emphasis> sp.)

Species of the macroalgae Caulerpa sp. are increasingly being observed in meadows of the endemic Mediterranean seagrass Posidonia oceanica, and in particular Caulerpa taxifolia, has been considered as an invasive species leading to seagrass decline. Studies have so far failed to reveal the underlying mechanisms of the success of the macroalgae, and here, we examine how biogeochemical changes of the environment associated to indigenous (Caulerpa prolifera) and non-indigenous (Caulerpa racemosa and C. taxifolia) species affect the habitat of P. oceanica. Two of the species (C. prolifera and C. racemosa) affect the sediment biogeochemical conditions by increasing organic matter pools, microbial activity, and sulfide pools of the sediments, and limited effects were found for C. taxifolia. Biomass of the macroalgae contributed to the extent of impacts, and high sulfide invasion into the seagrasses and regression of the meadow were pronounced at the location with the highest Caulerpa biomass. This suggests that Caulerpa invasion contributes to seagrass decline probably because Caulerpa thrives better than the seagrasses in the modified environment.     

Paleolimnological significance of observed distribution patterns of chrysophyte cysts in arctic pond environments

In a survey of 35 high arctic ponds, chrysophycean cysts were relatively more common in moss periphyton and epilithon habitats, than in surface sediment samples. The highest percentages of cysts relative to diatoms were found in the semi-aquatic mosses. Although chrysophytes are generally considered to be planktonic, periphytic taxa may be common in high latitudes. The ratio of diatom frustules to chrysophyte cysts in arctic sediment cores may be tracking different environmental variables than paleolimnologists may intuitively expect based on observations from more temperate regions.     

Global-Scale Relationships between Climate and the Dengue Fever Vector, Aedes Aegypti

Considerable interest exists inthe potential role climate may play in human healthissues, especially regarding the effect of climatechange on vector-borne disease. The Aedesaegypti mosquito, the principal vector for dengue,considered the most important vector-borne viraldisease in the world, is particularly susceptible toclimate variability and climatic change. Here wepresent a modeling analysis focusing on global-scaleassociations between climate and the development,potential distribution, and population dynamics ofAe. aegypti. We evaluate the model by comparingand contrasting model data with observed mosquitodensities. There is good agreement between theobserved and modeled global distribution of themosquito; however, the model results suggest thepotential for increased latitudinal distributionsduring warmer months. Seasonal fluctuations inmosquito abundance also compare well to observed data. Discrepancies possibly reflect the relatively lowresolution of the climate data and model output andthe inability of the model to account for localmicroclimate effects, especially in coastal areas.Future modeling efforts will involve study ofinterannual variability in mosquito dynamics.     

Sulfide intrusion in the tropical seagrasses Thalassia testudinum and Syringodium filiforme

Sulfur and oxygen dynamics in the seagrasses Thalassia testudinum and Syringodium filiforme and their sediments were studied in the US Virgin Islands (USVI) in order to explore sulfide intrusion into tropical seagrasses. Four study sites were selected based on the iron concentration in sediments and on proximity to anthropogenic nutrient sources. Meadow characteristics (shoot density, above- and below-ground biomass, nutrient content) were sampled along with sediment biogeochemistry. Sulfide intrusion was high in T. testudinum, as up to 96% of total sulfur in the plant was derived from sediment-derived sulfides. The sulfide intrusion was negatively correlated to the turnover of sulfides in the sediments regulated by both plant parameters and sediment sulfur pools. Sediment iron content played an indirect role by affecting sulfide turnover rates. Leaf production was negatively correlated with sulfide intrusion suggesting that active growth reduced sulfide intrusion. Sulfide intrusion was lower in S. filiforme (up to 44%) compared to T. testudinum consistent with a higher internal nighttime oxygen concentrations found for S. filiforme. When S. filiforme can take advantage of its ability to maintain high internal oxygen concentrations, as was the case on the USVI, it could increase its success in colonizing unvegetated disturbed sediments with potentially high sulfide concentrations.     

Distribution and bioturbation effects of the tropical alpheid shrimp Alpheus macellarius in sediments impacted by milkfish farming

The impact of milkfish fish pens on the distribution of the alpheid shrimp Alpheus macellarius was studied in the Bolinao area, Philippines. In addition, the impact of the alpheid shrimp on sediment biogeochemistry, including organic matter mineralization, nutrient and sulfur cycling was compared in sediments affected and unaffected by fish farming. The fish farming activity had negative impact on the distribution of shrimps by reducing the abundance up to 60% in the vicinity of the net pens, and to almost absence inside the net pens. At a farm site abandoned seven month prior to sampling the abundance of shrimps was still low and sediment pools of reduced sulfur high, indicating a slow benthic recovery. The shrimp had large physical impact on the sediments, and the mineralization rates were stimulated by the presence of the shrimp in pristine sediments, whereas the fish farm impacted sites showed high mineralization rates both in sediments with and without the presence of the shrimp suggesting that the organic matter enrichment stimulated the mineralization in these low-organic sediments, whereas the shrimp had less impact. Sulfate reduction rates and in particular pools of sulfides increased in the farm impacted sediments, and sulfate reduction was an important process for organic matter mineralization (>67%) in all sediments. Although the shrimp had minor effect on the organic matter mineralization and nutrient fluxes in the farm impacted sediments, they appeared to increase the oxidation of sulfides by increasing the zone of oxidized surface sediment and thereby improving the sediment conditions for recolonization after fish farming has ceased.     

Negative effects of blue mussel (Mytilus edulis) presence in eelgrass (Zostera marina) beds in Flensborg fjord,Denmark

The effect of blue mussel (Mytilus edulis) presence in eelgrass (Zostera marina) beds was studied from June 2004 to July 2005 in Flensborg fjord, Denmark. The field experiments were conducted at two stations, one with only Z. marina (Eelgrass station) present and one where M. edulis were present in the Z. marina beds (Mixed station). Zostera marina parameters were measured (growth of leaves, shoot density, leaf length, and nutrient content) in combination with epiphyte cover and sediment parameters (sulphate reduction rates, sediment nutrient fluxes, organic content, C, N and P content) to examine possible positive and negative effects of the mussels on eelgrass performance. The fluxes of ammonium from the sediments were stimulated at all sampling dates at the Mixed station, and possibly stimulated epiphyte growth at this station. Further ¹⁵N signals in epiphytes from the Mixed station suggested that excretion products from the mussels were important nitrogen sources at this station. Sulphate reduction rates were enhanced at the Mixed station and also sediment sulphide concentrations increased under mussel influence, which may have resulted in sulphide toxicity and decreased growth of Z. marina at this station. The study indicates that for Z. marina beds in Flensborg Fjord the effects of M. edulis in seagrass beds are primarily negative, and raises the question whether this leads to negative effects on the stability and expansion of Z. marina beds.     

Mid-Holocene hemlock decline and diatom communities in van Nostrand Lake, Ontario, Canada

Terrestrial ecosystem disturbances inferred from the fossil hemlock pollen decline (ca. 4,800 BP) and recovery (ca. 3,800 BP) affected van Nostrand Lake, including the diatom communities. Ecological models suggest the lake responded by eutrophying, reflecting higher nutrient influx resulting from increased erosion. A decline in lake productivity followed as the forest vegetation recovered and erosion slowed. Lastly, as the forest switched from early and middle successional species to mature species, lake productivity increased as erosion increased nutrient inflow, especially phosphorus. However, this eutrophication response was delayed or buffered, perhaps due to wetland development surrounding the lake. The lake never fully returned to its initial, pre-hemlock decline state, but oscillated between more eutrophic and more mesotrophic or oligotrophic stages, possibly reflecting other disturbances in the catchment and climatic change.