Danilo Calliari

Emergent neutrality drives phytoplankton species coexistence

The mechanisms that drive species coexistence and community dynamics have long puzzled ecologists. Here, we explain species coexistence, size structure and diversity patterns in a phytoplankton community using a combination of four fundamental factors: organism traits, size-based constraints, hydrology and species competition. Using a ‘microscopic’ Lotka–Volterra competition (MLVC) model (i.e. with explicit recipes to compute its parameters), we provide a mechanistic explanation of species coexistence along a niche axis (i.e. organismic volume). We based our model on empirically measured quantities, minimal ecological assumptions and stochastic processes. In nature, we found aggregated patterns of species biovolume (i.e. clumps) along the volume axis and a peak in species richness. Both patterns were reproduced by the MLVC model. Observed clumps corresponded to niche zones (volumes) where species fitness was highest, or where fitness was equal among competing species. The latter implies the action of equalizing processes, which would suggest emergent neutrality as a plausible mechanism to explain community patterns.

Competition Drives Clumpy Species Coexistence in Estuarine Phytoplankton

Understanding the mechanisms that maintain biodiversity is a fundamental problem in ecology. Competition is thought to reduce diversity, but hundreds of microbial aquatic primary producers species coexist and compete for a few essential resources (e.g., nutrients and light). Here, we show that resource competition is a plausible mechanism for explaining clumpy distribution on individual species volume (a proxy for the niche) of estuarine phytoplankton communities ranging from North America to South America and Europe, supporting the Emergent Neutrality hypothesis. Furthermore, such a clumpy distribution was also observed throughout the Holocene in diatoms from a sediment core. A Lotka-Volterra competition model predicted position in the niche axis and functional affiliation of dominant species within and among clumps. Results support the coexistence of functionally equivalent species in ecosystems and indicate that resource competition may be a key process to shape the size structure of estuarine phytoplankton, which in turn drives ecosystem functioning.

A trait-based approach to summarize zooplankton–phytoplankton interactions in freshwaters

Assessing zooplankton grazing on phytoplankton is crucial to understand, model, and predict the structure and dynamics of pelagic communities. Our hypothesis is that phytoplankton consumption by zooplankton in freshwater lakes can be well represented by clustering phytoplankton species into morphology-based functional groups (MBFG) and zooplankton species into broad taxonomic units: cladocerans, calanoid copepods, and rotifers. We characterized zooplankton potential grazing on MBFG based on an extensive literature review of experimental data including clearance and ingestion rates. Rotifers show greater potential grazing upon small- and medium-sized species (MBFG I and IV) and presented a Type III trait-based functional response. Cladocerans also show greater potential impact upon MBFG IV but a Type II response. Both groups maintained their respective feeding response regardless of the type of food available, indicating poor food selectivity. Copepods consumed different MBFGs, but a clear Type II pattern was observed when feeding on MBFGs V and VI. Prediction intervals indicated a greater variability in cladocerans’ and copepods’ response. This approach is a step to summarize and characterize grazing to the future quantification of ecosystem models. Further efforts should be done to include information about different larval stages and phytoplankton traits not directly related to morphology.

Salinity preferences and habitat partitioning between dominant mysids at the Rio de la Plata estuary (Uruguay)

Mysids are common and abundant organisms in estuaries and coastal waters, where they have a central role in the functioning of food webs. The Rio de la Plata )Rd1P) is a major estuary that affects biogeographic patterns of marine coastal organisms in the south-west Atlantic, but studies of the plankton in the RdlP are scarce. Knowledge of the mysid fauna in particular is very limited with a single species (likely alien, Neomysis americana), traditionally known to inhabit this 38,000 km 2 estuary, and a second one, Mysidopsis tortonesei, only recently reported. The purpose of this paper is to contribute with the knowledge of the mysid fauna of this ecosystem by exploring the spatial distribution and in situ salinity preferences of N americana and M. tortonesei in the mixohaline region of the Rdl1 during austral autumn, 2001. Both species showed clear and contrasting haline affinities that resulted in an almost perfect spatial segregation within the study area. Neomysis americana occurred preferentially in oligo- and meso-haline conditions (range 28. Temperature range was quite narrow and it was unlikely that it affected mysid distribution in the estuary. Temporal variability in observed patterns, as well as the role of biological interactions in shaping spatial distribution ofN americana and M. tortonesei within the estuary are subjects that deserve further attention in future studies.

Summertime herbivory and egg production by Acartia tonsaat the Montevideo Coast - Rio De La Plata

Abstract We present the first data on copepod feeding and production in the Río de 1a Plata, and discuss their correlation to the physical and biological environment. Sampling was performed at a protected area on the Montevideo coast where temperature, salinity and size fractionated chlorophyll aand phaeopigments were monitored weekly from January through February 2003, plus two additional observations in March and April. Acartia tonsaingestion was measured by gut fluorescence, and gut evacuation rates, egg production and egg hatching success were estimated using standard methods. Food availability was moderate/high, but quality appeared low as reflected by chlorophyll a:phaeopig-ment ratios <1. Ingestion correlated with both chl-a and pha, suggesting that A. tonsafed also on detrital matter. Egg production rates were low (1.4-7.5 eggs female1 d-1) — except for one exceptional record of ca. 88 eggs female-1 d-1 — indicating low productivity rates. Egg hatching success ranged from 30-94% and tended to increase during the study period. Overall results suggest that at the study site A. tonsafed on both phytoplankton and detritus, and that its production was limited by food quality.

Evaluation of Culex pipiens larvae control by cyclopoid copepods in an urban cemetery of Montevideo, Uruguay

has caused particular concern (Willat et al. 2003) as dengue is still an endemic disease in neighboring Brazil and Argentina. Continued use of pesticides for mosquito control is a common practice but tends to generate chemical resistance in addition to seriously harming the environment (Schrieber and Jones 2000). Biological control is a promising alternative for keeping vector populations in check. Laboratory (Rey et al. 2004, Micieli et al. 2002, Marten 1989) and field studies (Brown et al. 1991, Cabral 1993, Marten 1990, 1993, Nam et al. 2005) indeed support that cyclopoid copepods (Crustacea) can be effective controllers for different mosquito species. Assessment of endemic copepod species is necessary in order to develop a biological control approach. In Uruguay, local strains of

A multilevel trait-based approach to the ecological performance of Microcystis aeruginosa complex from headwaters to the ocean

The Microcystis aeruginosa complex (MAC) clusters cosmopolitan and conspicuous harmful bloom-forming cyanobacteria able to produce cyanotoxins. It is hypothesized that low temperatures and brackish salinities are the main barriers to MAC proliferation. Here, patterns at multiple levels of organization irrespective of taxonomic identity (i.e. a trait-based approach) were analyzed. MAC responses from the intracellular (e.g. respiratory activity) to the ecosystem level (e.g. blooms) were evaluated in wide environmental gradients. Experimental results on buoyancy and respiratory activity in response to increased salinity (0-35) and a literature review of maximum growth rates under different temperatures and salinities were combined with field sampling from headwaters (800km upstream) to the marine end of the Rio de la Plata estuary (Uruguay-South America). Salinity and temperature were the major variables affecting MAC responses. Experimentally, freshwater MAC cells remained active for 24h in brackish waters (salinity=15) while colonies increased their flotation velocity. At the population level, maximum growth rate decreased with salinity and presented a unimodal exponential response with temperature, showing an optimum at 27.5°C and a rapid decrease thereafter. At the community and ecosystem levels, MAC occurred from fresh to marine waters (salinity 30) with a sustained relative increase of large mucilaginous colonies biovolume with respect to individual cells. Similarly, total biomass and, specific and morphological richness decreased with salinity while blooms were only detected in freshwater both at high (33°C) and low (11°C) temperatures. In brackish waters, large mucilaginous colonies presented advantages under osmotic restrictive conditions. These traits values have also been associated with higher toxicity potential. This suggest salinity or low temperatures would not represent effective barriers for the survival and transport of potentially toxic MAC under likely near future scenarios of increasing human impacts (i.e. eutrophication, dam construction and climate change).

Revision of mysids (Crustacea: Peracarida: Mysida) zoogeographical distribution in the south-west Atlantic (0° to 40°S)

We revised the zoogeographical distribution of mysids in the south-west Atlantic, from northern Brazil to Argentina (08 to 408S), providing body length, and temperature and salinity range of occurrence for each species. All information concerning mysids from the south-west Atlantic was obtained from published/unpublished studies. Of currently 1128 mysids species described worldwide, only 31 were recorded in the south-west Atlantic which are distributed in 14 genera, 6 subfamilies and one family. This extremely low number of recorded species highlights the need for further studies focusing on mysids in the south-west Atlantic.

Trophic Ecology of the White Croaker ( Micropogonias furnieri Desmarest, 1823) and Rough Scad ( Trachurus lathami Nichols, 1920) Larvae in the Río de la Plata Estuary

This study describes the feeding ecology of white croaker (Micropogonias furnieri) and rough scad (Trachurus lathami) larvae in the Rio de la Plata Estuary (RPE) during a breeding season. The working hypotheses were that the coexistence of the species under study is favored by spatial segregation and divergence in mouth gape size, which mirrors their diet composition and trophic niche breadth (TNBs), based on prey size. Micropogonias furnieri was more abundant at the innermost stations, while T. lathami was most abundant at outermost stations of the estuary. However, their abundances were uncorrelated to environmental salinity and temperature. Micropogonias furnieri and T. lathami larvae showed differences in their gape growth patterns and in the type of preys ingested, which resulted in low dietary overlap. This did lead to differences in the TNBS and supported the prediction of the trophic differentiation hypothesis and could be interpreted as a mechanism to minimize food competition in the RPE ecosystem.

Near-Surface Biogeochemistry and Phytoplankton Carbon Assimilation in the Rio de la Plata Estuary

The Rio de la Plata estuary (RPE) is considered a highly productive ecosystem, but knowledge of its functioning is sparse, particularly at basal trophic levels. Direct measurements of primary production are scarce, and the mechanisms that drive biological production and biogeochemistry of nutrients and other key compounds are largely unknown. This review summarizes the current information available in published literature in standard journals and other sources and explores driving mechanisms for photosynthetic carbon assimilation (PCA). A database was compiled which includes photosynthetic rates and ancillary environmental variables, i.e. salinity; chlorophyll a (as surrogate for phytoplankton biomass); dissolved inorganic macronutrients N, P and Si; suspended particulate matter (SPM); and underwater light environment. Information gathered covers an extended time period but clustered into an early (1980–1987) and a more recent one (1999–2009). Data was unequally distributed between both periods; for example, PCA data exist only for the most recent period. Data indicate prevalence of high photosynthetic rates in the RPE (mean of 29.25 ± 22.61 mg C m−3 h−1). The concentration of nutrients, SPM and indicators of underwater light regime covered wide ranges of variability with remarkable differences between both time periods. All these variables were strongly affected by salinity, showing either a decreasing pattern of concentration from freshwaters to marine waters (e.g. nutrients) or from marine waters to freshwaters (i.e. better light conditions in marine-influenced waters). These results were valid irrespective of the period and of the light environment indicator considered (Kd, turbidity). PCA was highest at intermediate surface salinities (10–20), decreasing both towards fresh and towards marine areas. Observed variability patterns of nutrients, SPM, light regime and carbon assimilation along the salinity gradient were consistent with mechanisms predicted by the theory developed for turbid estuaries, which propose that PCA is regulated by inverse horizontal gradients of light and nutrients.