Effects of nutrients and fish on periphyton and plant biomass across a European latitudinal gradient

Replicated, factorial mesocosm experiments were conducted across Europe to study the effects of nutrient enrichment and fish density on macrophytes and on periphyton chlorophyll a (chl-a) with regard to latitude. Periphyton chl-a densities and plant decline were significantly related to nutrient loading in all countries. Fish effects were significant in a few sites only, mostly because of their contribution to the nutrient pool. A saturation-response type curve in periphyton chl-a with nutrients was found, and northern lakes achieved higher densities than southern lakes. Nutrient concentration and phytoplankton chl-a necessary for a 50% plant reduction followed a latitudinal gradient. Total phosphorus values for 50% plant disappearance were similar from Sweden (0.27 mg L⁻¹) to northern Spain (0.35 mg L⁻¹), but with a sharp increase in southern Spain (0.9 mg L⁻¹). Planktonic chl-a values for 50% plant reduction increased monotonically from Sweden (30 μg L⁻¹) to València (150 μg L⁻¹). Longer plant growing-season, higher light intensities and temperature, and strong water-level fluctuations characteristic of southern latitudes can lead to greater persistence of macrophyte biomass at higher turbidities and nutrient concentration than in northern lakes. Results support the evidence that latitudinal differences in the functioning of shallow lakes should be considered in lake management and conservation policies.     

Impacts of hydroperiod on growth and survival of larval amphibians in temporary ponds of Central Pennsylvania,USA

The effects of variable hydroperiod (three levels) and initial density of amphibians (two levels) on survival, growth rate, and time to and mass at metamorphosis were studied for wood frogs (Rana sylvatica), Jefferson salamanders (Ambystoma jeffersonianum), and spotted salamanders (A. maculatum). Experiments were carried out in 260-1 mesocosms set up outdoors in a forest. These pond simulations were designed to mimic conditions that occur in palustrine temporary wetlands in central Pennsylvania. No animals reached metamorphosis in the short hydroperiod (56 days). However a greater proportion (66%) of tadpoles of R. sylvatica survived to the end of the 56-day, treatment than the 84- or 158-day treatments (29 and 14%, respectively), from which all survivors metamorphosed. In contrast, neither of the salamanders metamorphosed by 84 days; survival to metamorphosis at 158 days was 15% for A. jeffersonianum and 10% for A. maculatum. Average instantaneous growth rates for A. jeffersonianum decreased with each increase in hydroperiod. Growth of R. sylvatica was greater in the 56-day hydroperiod than in hydroperiods of 84 or 158 days. Initial amphibian density had no effect on growth or survival of any species. It appears that salamander larvae were predatory on tadpoles, since survival of R. sylvatica was negatively correlated with survival of A. jeffersonianum in 84-day treatments and with growth of A. maculatum in 158-day treatments.     

The effects of variable nutrient additions on a pond mesocosm community

The effects of nutrient additions on aquatic systems have been frequently studied. Typically, these studies report an increase in algal biomass and a decrease in species diversity in response to nutrient increases. However, it is not clear why comparable aquatic communities respond differently to nutrient additions of similar magnitudes. We tested the effects of the rate and amount of nutrient load on community structure in 760 l mesocosms; treatments manipulated the total amount of nutrients that entered an aquatic system (small versus large load) and the temporal pattern in which these nutrients entered the system (annually, monthly, or weekly). We found that the effects of the loading rate of nutrients were at least as important as the total amount of the nutrients for several response variables. Although these effects were manifested in several ways, the response to the different rates was most prominent within groups of the primary producers, which showed large shifts in composition and abundance. Handling editor: L. M. Bini     

Growth and nutrient absorption of two submerged aquatic macrophytes in mesocosms, for reinsertion in a eutrophicated shallow lake

Aquatic macrophytes play a central role in preserving the ecological equilibrium of shallow lakes and in the restoration of eutrophic lakes that have switched to phytoplankton-dominated turbid water. Massaciuccoli Lake, a shallow lake located along the Tuscan coast in Italy, has shown a constant and progressive simplification of the submerged plant community, for anthropogenic reasons, leading, in recent years, to turbid water. The growth and nutrient absorption capability of two macrophyte species, Myriophyllum verticillatum L. and Elodea canadensis Michaux, in the lake was investigated, with the prospect of a future lake restoration programme centred on their replacement. Mesocosm experiments were conducted to monitor the plant growth and nutrient (NO₂⁻, NO₃⁻, NH₄⁺, Ntot, PO₄³⁻, Ptot) content in the plant dry matter and water at the beginning and at the end of the trial. Bacterial activity was analysed in the water in order to verify the possible nutrient absorption contribution by organisms other than plants. Both M. verticillatum and E. canadensis showed satisfactory growth and nutrient reduction in the water body. Moreover, their different growth patterns suggested that optimal replacement can be performed with their introduction in two steps, starting with M. verticillatum, which shows the best capacity to colonise the aquatic environment, due to its tendency towards lengthening.     

Predominant growth of Alcanivorax during experiments on "oil spill bioremediation" in mesocosms

Mesocosm experiments were performed to study the changes on bacterial community composition following oil spill in marine environment. The analysis of 16S crDNA revealed a shift in the structure of initial bacterial population that was drastically different from that one measured after 15 days. The results showed that, after 15 days, bacteria closely related to the genus Alcanivorax became the dominant group of bacterial community in petroleum-contaminated sea water nitrogen and phosphorus amended. This suggested that these bacteria played the most important role in the process of bioremediation of oil-contaminated marine environments.     

Grazing effect of the invasive reef-forming polychaete Ficopomatus enigmaticus (Fauvel) on phytoplankton biomass in a SW Atlantic coastal lagoon

In this work we evaluate the potential grazing impact of the invasive reef-forming polychaete Ficopomatus enigmaticus in a SW Atlantic coastal lagoon (Mar Chiquita coastal lagoon; 37° 40' S, 57° 23' W; Argentina). This gregarious species feeds on suspended detritus and phytoplankton. Given the large area dominated by reefs, suspension feeding by this species might reduce the lagoon phytoplankton concentration and even hinder local eutrophication. To evaluate this hypothesis in situ replicated mesocosm experiments were performed in spring 2005 and, summer and winter 2006. Mesocosms enclosing reefs and without reefs were installed and grazing intensity was measured as the difference in chlorophyll a concentration and turbidity between the reef-treatment and the treatment without reefs. Reefs of F. enigmaticus decreased the mean chlorophyll a concentration, more during summer (56% decrease) than spring (25% decrease) and winter (19% decrease). Reefs also decreased water turbidity during summer (54% decrease) and spring (23% decrease). While previous studies indicate that the physical structure of these reefs alters water flow increasing water turbidity, our evidences show that their suspension-feeding activity can counteract this effect. Water turbidity was positively correlated with chlorophyll a concentration, which suggests that phytoplankton grazing by F. enigmaticus decreased light attenuation through the water column, with a potential for enhancement of benthic primary productivity. Therefore, our results suggest that grazing by this polychaete affects overall estuarine primary production as well as the relative importance of planktonic and benthic carbon sources to higher trophic levels.     

Cannibalistic interactions in two co-occurring decapod species: Effects of density, food, alternative prey and habitat

Cannibalism is a potentially important factor in the regulation of populations in a range of habitats. The intensity of this biotic factor may be determined by both intra and interspecific interactions. Cancer pagurus and Porcellana platycheles are two co-occurring decapods on Atlantic rocky shores. In laboratory mesocosms, we investigated intra and intersize class cannibalistic and interspecific predatory behaviours in those species. We addressed the effects of prey and predator densities, food, starvation, alternative prey and habitat type. No agonistic behaviour was noted in P. platycheles, suggesting a non-aggressive co-existence between gregarious individuals. Predation of C. pagurus on P. platycheles was intense, possibly accounting for the spatial segregation observed in the natural environment. Cannibalism among C. pagurus juveniles was low and only on vulnerable prey (i.e. at moulting), suggesting a non-aggressive co-existence among juveniles. However, intersize class cannibalism in C. pagurus was intense (ontogenetic shift), possibly reflecting the juvenile-adult segregation in the natural environment. Prey and predator densities, food and habitat type strongly influenced this behaviour. Possible interference among cannibals was noted, with lower prey consumption at high predator density. Food supply alone had more effect on cannibalistic rate than did alternative prey (P. platycheles) and predator starvation. Structurally complex habitats (small pebble and Fucus serratus habitats) yielded higher prey survival than the sandy habitat, and the behaviours of both prey and cannibals reflected the small-scale spatial distribution of individuals in the wild. Intersize class cannibalism and interspecific agonistic relationships may account for the intertidal distribution of crab species at low tide.     

Effect of Watering and Soil Moisture on Mercury Emissions from Soils

This paper presents data from experiments that measured Mercury (Hg) flux as a function of water addition and subsequent soil drying, and maintenance of soil water content over time utilizing small dynamic gas exchange chambers and large mesocosms. When soil surfaces were dry and water was added at an amount less than that necessary to saturate the soil an immediate large (relative to dry soil flux) release of Hg occurred. Diel Hg emissions from soils, unenriched (0.02 μg g⁻¹) and enriched (3 μg g⁻¹) in Hg and wet below saturation, were significantly elevated above that occurring from dry soils (2–5 times depending on soil water content) for weeks to months. Enhancement of emissions from wet soils in direct sunlight were greater than that from soils shaded or in the dark suggesting that a synergism exists between soil moisture and light. When soils were watered to saturation Hg emissions were suppressed or remained the same depending on the degree of saturation. It is hypothesized that the addition of soil water in amounts less than that necessary to saturate the soil surface results in an immediate release of elemental Hg from soil surface as the more polar water molecule out competes Hg for binding sites. As the water moves into the soil, Hg adsorbed to soil particles is desorbed into soil gas and dissolved in the soil water. The process of evaporation facilitates movement of Hg as mass flow to the soil surface where it is made available for subsequent release. The latter is hypothesized to be an important process by which Hg is recharged at the soil–air interface.     

A mesocosm study of Phaeocystis globosa (Prymnesiophyceae) population dynamics: II. Significance for the microbial community

The impact of Phaeocystis globosa population decline on the microbial community was studied during a mesocosm experiment, with irradiance regime and inorganic N:P ratios (4, 16, and 44) as controlling factors. Heterotrophic bacterial activity was closely related to enhanced (viral) lysis rates of P. globosa cells and disintegration of the colonies. Up to 85% of the bacterial C demand could be supplied by P. globosa-specific cellular C release. The bacterial populations with high DNA content became dominant (>70% of total). The bacterial community showed a rapid shift in composition to take advantage of the changing conditions during the demise of the P. globosa bloom. Members of the Alphaproteobacteria and the Bacteroidetes group emerged directly upon bloom decay. Multidimensional scaling analysis in conjunction with DGGE fingerprinting implied that clustering was more related to the availability of organic carbon (the collapse of the P. gobosa bloom) than to the nature of the phytoplankton growth-controlling nutrient. Reduced irradiance delayed the development of the P. globosa population and subsequently changes in the bacterial community composition. Disintegration of P. globosa colonies resulted in the formation of transparent exopolymeric particles (TEP) and aggregates, more so under P-depletion than under N-deficient conditions. The colonial matrix transformed into big aggregates under P-depleted conditions but remained largely as ghost colonies under N-depleted conditions. In the mesocosm with initial nitrogen and phosphorus supplied in the Redfield ratio, features intermediate to conditions with either N- or P-depletion were observed. It was hypothesized that TEP affected microbial population dynamics directly through bacterial colonization and indirectly through scavenging of predators and viruses.     

Effects of salinity on leaf growth and survival of the Mediterranean seagrass Posidonia oceanica (L.) Delile

The main aim of this study was to estimate the effects of salinity variation on the Mediterranean seagrass Posidonia oceanica (L.) Delile and its attached epiphytes. Leaf growth and survival of this plant were tested in several short-term (15 days) mesocosms experiments under controlled conditions between February 2001 and November 2001. Plants collected from shallow meadows at Alicante (SE Spain), with an ambient salinity of 36.8-38.0 psu, were placed in tanks of 300 L with an additional overhead light and exposed to different salinity treatments (ranging from 25 to 57 psu) during 15 days. To estimate the mortality and growth recuperation, in some experiments shoots were returned to control salinity (38 psu). Leaf growth was measured in the laboratory where epiphytic fauna and flora were removed from leaves, with a razor blade, to determine their biomass.P. oceanica was negatively influenced by increased salinity. Shoots showed a significant decrease in growth and survival, whereas epiphyte biomass did not show a clear response because of their high variability. Maximum leaf growth occurred between 25 and 39 psu. In addition, plants suffered considerable mortality at salinities above 42 psu and below 29 psu, with 100% mortality at 50 psu. In salinities between 39 and 46 psu, surviving plants were able to regain their original growth rate when returned to normal seawater salinity (38 psu). These results suggest that P. oceanica is one of the most sensitive seagrasses to salinity increments it is more tolerant to salinity reductions (25.0-36.4 psu), perhaps due to the terrestrial origin of seagrasses.