Nancy Correa

Dispersion and Ecological Impact of the Invasive Freshwater Bivalve Limnoperna fortunei in the Río de la Plata Watershed and Beyond

Limnoperna fortunei is a freshwater bivalve that invaded South America through Río de la Plata estuary in 1989 and has since become a major macrofouling pest. Along the Paraná-Paraguay waterway, which hosts intense boat traffic, L. fortunei has moved upstream at an average rate of of 250 km per year. In contrast, along the Uruguay river, where boat traffic is restricted to the lowermost 200 km section, upstream colonization is almost 10-times slower. This suggests that attachment to vessels is by far the most important dispersion mechanism. It is suggested that the Amazon, Orinoco and Magdalena basins are under high risk of invasion by this mussel, especially through their estuarine gateways. All South American basins host innumerable water bodies with favorable conditions for L. fortunei’s colonization. Known ecological tolerance limits of the mussel also suggest that it may colonize much of the area from Central America to Canada, including waters that due to their low calcium contents, high temperature and pollution levels, and low oxygen are inadequate for the survival of Dreissena polymorpha. Despite it’s remarkable geographic expansion and its extremely high population densities, L. fortunei’s ecological effects have received very little attention so far. It is suggested that the 2.4-fold increase in Argentine landings of freshwater fish between 1992–1993 and 2000–2001 may be associated with the introduction of this prey species.

Feeding selectivity of Corbicula fluminea(Bivalvia) on natural phytoplankton

Phytoplankton in water samples from the lower delta of the Paraná river (Argentina) and in the gut contents of a local populaton of the Asiatic clam Corbicula fluminea was identified and counted monthly between April 1992 and March 1993. Relative abundances of algal species were generally similar in the medium and in the stomachs of clams, suggesting the absence of feeding selectivity on the basis of taxonomic group or size-class. Diatoms made up a slightly but consistently larger (yet not significantly different) proportion of the diet of C. fluminea than of the plankton; this pattern is attributed to enhanced preservation of algal siliceous frustules, rather than to feeding selectivity. It is suggested that non-selective feeding by the bivalve is a response to the overall scarcity of food and, probably, to food collection by a combination of deposit and filter feeding.

Planktic foraminifera from the southwestern Atlantic (30 °–60 °S): species-specific patterns in the upper 50 m

Abstract Planktic foraminifers were studied in 96 samples collected in the southwestern Atlantic (30 °–60 °S, along 53 °W) in November 1993, mainly from depths between 0 and 50 m. Very high proportions of juveniles (unidentified) were present throughout the area, especially north of 37 °S, where they accounted for up to 70–80% of all shells recorded. For most species no clear vertical specific stratification was detected in the 0–50 m layer. Zoogeographic grouping of the 18 species identified allowed the defining of 5 distinct zones along the transect: Subtropical (north of 31 °S, 80% warm water individuals); Warm-Transitional (34 °–37 °S, 35% warm water); Transitional (37 °–49 °S, 99% cold water); Subantarctic (49–55 °S, 100% cold water); and Antarctic (south of 56 °S, 100% cold water). Boundaries between foraminiferal assemblage zones are in good agreement with hydrological fronts described for the area. Comparison of the present data with planktic collections from the North Atlantic show large differences in the proportions of various taxa. In the 14–24 °C range, G. bulloides is much more abundant in the northern hemisphere than in the southern one, whereas G. quinqueloba G. inflata and G. rubescens show the opposite trend. On the other hand, temperature-related percentage contributions within the 14 °–24 °C range indicate that the preferred thermic regimes of the 9 species considered are remarkably similar in the North and South Atlantic collections compared. The southernmost planktic distributional ranges of selected warm water taxa are roughly coincident with those established previously on the basis of surface sediments, disagreements being chiefly attributable to selective dissolution on the bottom. In contrast, at the bottom percentages of cold water foraminifers ( G. bulloides G. pachyderma ) are significantly enhanced with respect to their planktic populations, and their sedimentary northward limits extend well beyond their maximum ranges in the upper-layer plankton. It is suggested that submergence of these cold water species and northward displacement at subsurface depths is chiefly responsible for the mismatching patterns observed. Dissimilar species proportions in both hemispheres and plankton-sediment uncoupling can conceivably engender erroneous conclusions when distributional data are used for paleoecologic reconstructions with the aid of numerical analyses such as the Imbrie-Kipp transfer-function technique.

Ecosystem impacts of the invasive bivalve Limnoperna fortunei (golden mussel) in South America

We summarize current knowledge on the effects of the invasive Asian bivalve Limnoperna fortunei (introduced in South America around 1990) on local biota. Limnoperna modifies nutrient concentrations and decreases concentrations of particulate organic matter in the water column (including phytoplankton and zooplankton), thus enhancing light penetration and stimulating growth of periphyton and macrophytes. Selective grazing and modification of the N:P ratio are responsible for strong enhancements of toxic cyanobacterial blooms. Limnoperna beds significantly enhance the numbers, biomass, and diversity of practically all accompanying invertebrates. The mussel’s planktonic larvae represent an important food item for the larvae of 18 fish species, while juveniles and adults are consumed by at least 50 fish species. Limnoperna is the first and only abundant benthic filter-feeding animal in South American continental waters. The fact that it intercepts and retains in the freshwater lotic domain particulate organic matter that would otherwise be swept into the sea must represent an important energetic subsidy, but the ecosystem-wide consequences of this trophic shift have not yet been addressed. Comparison with the impacts of the zebra mussel in Europe and North America suggests important differences.

Marine zooplanktonic diversity: a view from the South Atlantic

Abstract Approximately 7000 marine zooplanktonic species have been described so far for the World Ocean; in the South Atlantic the presence of 40% of these has been confirmed, and an additional 20–30% are expected to be recorded in the future. The overall number of described species is very low when compared with other communities, and yet it may not be too far from the final, complete inventory. Very ample geographic distributional ranges, compositional similarity between the major oceanic basins, and declining species description rates suggest that the undescribed fraction of marine zooplankton is nowhere as large as those suggested for the biosphere as a whole. It is anticipated that the highest proportions of new species will be among the groups associated with the sea-floor (meroplanktonic and benthopelagic forms). However, the fact that a high proportion of the marine zooplanktonic species has already been described does not imply that the corresponding taxonomic systems are adequate and that our understanding of this community is better than that of others where undiscovered species are still the overwhelming majority. For most marine zooplanktonic species we have extremely scarce biological and ecological information. Furthermore, the taxonomy of several quite speciose groups is in such a state of disarray that synonyms by far outnumber “good species”.

Planktonic equatorial diversity troughs: fact or artifact? Latitudinal diversity gradients in Radiolaria

In contrast to the classical notion of an increasing biodiversity from the poles to the equator, a number of studies concluded that the diversity of marine species is highest at the middle latitudes, and decreases at the equator. Using a worldwide database critically compiled from 72 surveys (307 species, 4,807 water column and surface sediment samples), we analyzed the latitudinal gradients in species richness (LGSR) of a highly diversified group of marine holoplanktonic protists, the polycystine Radiolaria. Species richness values were corrected for uneven sample coverage and sample size, and contrasted with gradients in 11 environmental variables. Radiolarian species richness decreases from the equator to the poles both in the water column and in the surface sediments and is tightly coupled with temperature throughout the entire thermal range of marine waters. In the tropical Pacific Ocean, a conspicuous east-west gradient in diversity is also associated with temperature. Globally, diversity is negatively correlated with mean annual concentrations of nutrients (N, P, Si) and chlorophyll a. Disagreements with results reported for many other oceanic plankton may stem from the reduction of 3D distributional patterns onto 2D or 1D spaces, to the intermittent mixing of Subtropical and Subpolar species at the middle latitudes, and to a Mid-Domain Effect. The fact that radiolarian LGSR do not show this drop at the equator is partly due to methodological and database-related differences, and probably also in part a reflection of taxon-specific traits.

Limnoperna Fortunei Colonies: Structure, Distribution and Dynamics

Settlement of new recruits of Limnoperna fortunei occurs preferentially on areas already colonized by conspecifics, and on surfaces with well-developed periphytic biofilms. Hard substrata (immobile rocks, wood) are preferred by the mussel, but colonization can also take place on muddy areas stabilized by roots or fibrous debris, on floating and submerged plants, and on mussel shells, crustaceans, etc. Colonization starts in crevices, angles and other sites inaccessible to large predators, but it often extends over open areas as well. Mussel beds rarely exceed 7–10 cm in thickness, with most adults being at least partially attached to the substrate. Juveniles often settle on larger shells. Densities of over 200,000 ind./m2 have been reported occasionally, but such high numbers are invariably dominated by specimens 5-7 mm) are usually below 10,000 ind./m2. The only site where densities were estimated over an entire water body, the reservoir Embalse de Rio Tercero, yielded an average of 959 ind./m2. Mussel colonies are usually most abundant and dense along the coastal fringe, where rock outcrops are common. Deeper areas are covered with clay and silt, and are therefore unfit for mussel colonization. Data at hand are still insufficient for describing multiannual trends in mussel abundance in South America; however, ancillary evidence suggests that, after having peaked 7–10 years after introduction, densities have been waning. Size structure of individuals in mussel colonies depends strongly on the time of the year. During periods of peak recruitment (spring to late summer) juveniles 2 mm in length can represent >90% of the population, whereas during the winter they normally account for 10-15%.

Nutrient Recycling, Phytoplankton Grazing, and Associated Impacts of Limnoperna fortunei

Laboratory and field experiments indicate that the presence of Limnoperna fortunei decreases concentrations of particulate organic matter and increases ammonia, nitrate, and especially phosphate. Long-term series of field data partially confirm these results. After having been colonized by the mussel, a 47 km2 reservoir developed higher concentrations of ammonia and phosphates, a higher P:N ratio, more transparency, less seston, and less phytoplankton and primary production. Phytoplankton clearance rates by the mussel vary widely, suggesting that “normal” values for adult organisms are around 100 mL/ind./h, or ca. 2–4 mL/mg DW/h. Data on grazing selectivity are inconclusive, but seem to indicate highest impacts on small (< 1 mm) particles. Large plankton are negatively selected, but they may account for greater proportions of total biomass in the diet. Studies on consumption of toxic cyanobacteria yield conflicting results, but large golden mussel populations significantly enhance blooms of colonial Microcystis spp. through changes in nutrient availability, size-selective grazing, promotion of colony formation, and reduced grazing of toxic cells. These toxic blooms, in turn, suppress reproduction of the mussel, most probably killing the larvae. Growth of periphyton and aquatic macrophytes are enhanced significantly by the golden mussel.

Reproductive Output and Seasonality of Limnoperna fortunei

Young Limnoperna fortunei mature sexually from 5–6 to ~15 mm. The species is generally dioecious, with approximately equal numbers of males and females and very small (< 0.6 %) proportions of hermaphrodites. The gametogenic cycle has been described for both Asian and South American populations, recognizing between four and five reproductive phases. Gonadal cycles based on histological sections yielded somewhat dissimilar results for different areas. In Hong Kong, two yearly peaks in reproductive output were detected. In South America, mature sperm and ova have been recorded year round and several irregularly spaced spawning events have been observed, as well as more or less continuous breeding punctuated by peaks in spring and at the end of the summer. Reproductive studies based on changes in the abundance of larvae in the water column have been carried out in South America and in Japan. In tropical and subtropical South America, larval output is more or less continuous for 6–10 months of the year, often with a major peak in spring–early summer, and a smaller one in the late summer–autumn. In Japan, at considerably lower water temperatures, larval production is limited to 1–2 months centered around summer. Apparent disagreements between results based on histological data and on larval counts stem from the fact that while the latter integrate the reproductive output of extensive mussel beds dispersed over large areas, histological evidence pinpoints with high precision the ripening and spawning of isolated mussel clusters. Aside from water temperature, several other factors (pH, salinity, dissolved oxygen, suspended solids, chlorophyll a, flood–drought cycles) have been proposed as reproductive triggers, but actual associations have not been demonstrated. Peak larval densities can exceed 20,000 ind./m3, but, normally, values range around 6000 ind./m3, showing major fluctuations within short periods, as well as changes as a function of time elapsed post colonization, and availability of substrata suitable for adult occupation. Microcystin-producing cyanobacterial blooms can kill L. fortunei larvae.