Oscar Iribarne

Eutrophication and macroalgal blooms in temperate and tropical coastal waters: nutrient enrichment experiments with Ulva spp.

Receiving coastal waters and estuaries are among the most nutrient-enriched environments on earth, and one of the symptoms of the resulting eutrophication is the proliferation of opportunistic, fast-growing marine seaweeds. Here, we used a widespread macroalga often involved in blooms, Ulva spp., to investigate how supply of nitrogen (N) and phosphorus (P), the two main potential growth-limiting nutrients, influence macroalgal growth in temperate and tropical coastal waters ranging from low- to high-nutrient supplies. We carried out N and P enrichment field experiments on Ulva spp. in seven coastal systems, with one of these systems represented by three different subestuaries, for a total of nine sites. We showed that rate of growth of Ulva spp. was directly correlated to annual dissolved inorganic nitrogen (DIN) concentrations, where growth increased with increasing DIN concentration. Internal N pools of macroalgal fronds were also linked to increased DIN supply, and algal growth rates were tightly coupled to these internal N pools. The increases in DIN appeared to be related to greater inputs of wastewater to these coastal waters as indicated by high δ15N signatures of the algae as DIN increased. N and P enrichment experiments showed that rate of macroalgal growth was controlled by supply of DIN where ambient DIN concentrations were low, and by P where DIN concentrations were higher, regardless of latitude or geographic setting. These results suggest that understanding the basis for macroalgal blooms, and management of these harmful phenomena, will require information as to nutrient sources, and actions to reduce supply of N and P in coastal waters concerned.

POSITIVE PLANT-ANIMAL INTERACTIONS IN THE HIGH MARSH OF AN ARGENTINEAN COASTAL LAGOON

Although plant-plant facilitations have been shown to be important struc- turing forces in salt marshes, less attention has been given to the potential role played by plant-animal facilitations in structuring these communities. We used a combination of sampling and field experiments to evaluate the effect of microenvironmental changes pro- duced by plant cover on the distribution of the burrowing crab Chasmagnathus granulata, a dominant macroinvertebrate of high marshes of the southwestern Atlantic coast. Four questions were explored. Is there a relationship between the spatial distribution of C. granulate and the spatial distribution of rooted macrophytes or distance from the marsh edge? How important is plant cover for the establishment and survival of crabs in the high marsh? Does plant cover affect critical physical variables for crab establishment? How important are environmental conditions for the survival of crabs in the high marsh? Surveys of the marsh surface showed that: (1) there was a strong relationship between the presence of plant cover and the spatial distribution of Chasmagnathus granulate in the high marsh, and (2) both mean crab size and burrow density decreased from the marsh edge to high-marsh levels. By shading the substrate, live plants and experimental plant mimics were found to be equally efficient at buffering high temperature, dehydration, and soil hardness in the high marsh. Experimental amelioration of these harsh physical con- ditions led to higher crab densities. Crab burrows also buffered harsh environmental con- ditions, diminishing surface air temperature from -460C to 230C. Finally, tethering ex- periments showed that stressful heating in the high marsh is lethal for crabs, and that plant cover is crucial for the establishment and long-term success of crabs in the high marsh. No predation was observed in tethering experiments. Our results suggest that plant cover is largely responsible for determining the spatial distribution of this dominant crab in the high marsh through facilitation. Thus, our work shows that plant-animal facilitations as well as plant-plant facilitations are important struc- turing forces in salt marsh communities.

Ecological Processes at Marine Fronts

Marine fronts are part of the structural complexity of the sea; they are narrow boundaries separating different water masses. Fronts are caused by diverse forcing and occur throughout the world ocean at several spatial and temporal scales. Unlike terrestrial ecotones, they show high biological production, affecting pelagic and benthic organisms of all trophic levels; consequently they are important for fisheries. Solar energy stimulates biological production in the entire biosphere, but in the sea it needs to be complemented by auxiliary energy to replenish plant nutrients; a significant quantity of mechanical energy becomes available for biological production at fronts. Global change is redistributing auxiliary energy in the oceans; consequently fronts are ideal sites for early monitoring of global change effects. Fronts also provide retention mechanisms for plankton in the highly dispersive marine environment; and become landmarks and beacons important for migrations or meeting of some species in a traceless realm.

Biology of Fronts

Vertical movements that bring nutrient-rich waters into the well-lit surface layers are at the base of the biological production of marine fronts; phytoplankton show strong positive reaction to such nutrient enrichment. The high primary production generated is transferred then to higher trophic levels reaching top predators, and also benthic organisms. High nutrient supply promotes the growth of large-sized phytoplankton, and consequently the development of shorter and more efficient food webs at fronts. Moreover, large-sized phytoplankton sinks relatively fast, increasing the food supply to benthic assemblages. The largest and more stable fronts are recognized as biogeographic boundaries, and those having lesser spatial scales or persistence exert their influences at finer spatial scales (ecoregions, assemblages). In most of the cases fronts do not appear to be absolute barriers, but are leaky boundaries. The most accepted effects of fronts on biodiversity are their impacts on divergences in species composition (β-diversity; assemblages), while their effects on absolute measures of biodiversity seem to be contradictory. Fronts result typically spawning grounds for species laying planktonic eggs. They offer adequate conditions for the development of the early life stages (abundant food; suitable physical-chemical ranges), and the possibility for eggs and larvae to be retained near the front, both passively or by coupling vertical migrating behavior to frontal circulation. Adult animals migrate to take advantages of seasonal habitats; migrants could utilize fronts as marks or paths to guide them in the highly dispersive and traceless pelagic realm. Animals may respond to physical-chemical gradients and/or prey abundance to find their way along the migration routes.

Effect of an invasive filter-feeder on the zooplankton assemblage in a coastal lagoon

Depletion of phytoplankton biomass by the introduced reef-forming polychaete Ficopomatus enigmaticus has previously been observed in the Mar Chiquita lagoon (37840′S 57823′W; Argentina), but the effect of polychaetes on the higher trophic levels is still unknown. To evaluate the effect of this polychaete on the zooplankton assemblage, replicated mesocosm experiments (N 1⁄4 10) were performed during spring, summer and winter. Mesocosms with reefs and without reefs were installed and grazing intensity and the effect on the zooplankton assemblage by the polychaetes were assessed. Our results show that the reefs of F. enigmaticus generate minor changes in overall composition of zooplankton assemblage. Although the structure of the zooplankton assemblage was different between seasons, the impact of the reefs was not significant in any of them. There was no relationship between the decline of food resource by grazing and changes in the structure of the zooplankton assemblage. Thus, contrary to our hypothesis, the grazing impact of the invasive polychaete on the biomass of primary producers did not generate cascading effects to higher trophic levels. However, changes in some components of the zooplankton assemblage (e.g. cladocerans) clearly show that the reefs of F. enigmaticus have the potential to affect the structure of the zooplankton community. The lack of data of community composition and abundance of zooplankton before the invasion limits the understanding of how this polychaete might have affected the structure and abundance of the zooplankton of this lagoon. Nevertheless this work suggests that these changes may not be so significant.

Comparisons of Fronts with Other Boundaries at Sea

There are boundaries in the ocean which are not fronts, such as the pycnocline, the interface of water with the air, the sediments or the ice. These boundaries pose distinctive biotic and abiotic conditions and are the preferred living space for certain groups of organisms. Due to the gravitational stratification, vertical scales in the sea are highly compressed relative to horizontal scales, so pycnoclines, and the interfaces between water and sediments; ice; and the atmosphere are all nearly horizontal edges having much larger horizontal scales than typical fronts. Most of these interfaces are layers of greatly reduced flows, weakening plankton dispersion. Life tends to congregate at boundaries and such non-frontal interfaces are places of locally elevated biological activity. However, non-frontal interfaces lack mechanisms that persistently bring nutrients to promote phytoplankton production. Fronts concentrate more biological productivity in narrower places, and their dynamics appear as more complex, characterized by intense three dimensional flows. Such complexity could explain the wider range of ecological properties of fronts compared to other interfaces.

Management and Conservation of Marine Life

Some species of commercial value, mainly pelagic fishes or squids, display fidelity to localized high-productivity fronts. Aggregations of organisms in predictable places facilitate fishing operations; moreover fishermen may easily detect certain kinds of fronts (e.g. thermal fronts) by employing satellite information, improving fisheries’ efficiency. Abundance of some benthic valuable resources (e.g. scallops) increase in fronts as well, forming dense and profitable beds. Species having little or no commercial value also concentrate at fronts; consequently, interactions between fisheries and vulnerable or endangered species are amplified at fronts: high bycatch rates of large pelagic sharks, sea turtles, marine mammals, and seabirds characterize some fronts. Fronts also exhibit a potential to concentrate several types of pollutants (e.g. plastics; oil; heavy metals) at their surface convergence and in sediments, thus endangering species that make use of the fronts. Because of potential dangers for marine life, fronts may be considered as valuable candidates for the implementation of protected areas. The ocean’s storage of carbon and ability to regulate atmospheric carbon dioxide is crucially dependent on primary production. Although high phytoplankton standing stocks do not necessarily imply CO2 sequestration, it has been shown that at least some fronts do present this type of biogeochemical response. It is expected that climate change will affect several of the physical forcing processes that generate and maintain fronts. Variations in the intensity of such forcing will affect key ecological processes associated with fronts. Because fronts depend on different forcing, it is expected that the impact and speed of climate change will vary among frontal types and geographical regions. Fronts play a significant role in the ecology of seas, and their forcing and properties are likely to change in response to climate change. Thus, it is suggested that fronts would be ideal sites for early monitoring and assessment of the dynamics of global state variables.

Comparisons of Fronts with Terrestrial Boundaries and the “Ecotone” Concept

Marine fronts possess most of the main characteristics of ecotones, as defined for land ecosystems, so fronts can be considered as true ecotones. In addition to land ecotones features, marine fronts are characterized by high biological productivity, a property rarely (if ever) reported for terrestrial ecotones. Moreover, concentration and retention of small planktonic forms in marine fronts plays a much more important ecological role than analogous processes on land. Both high biological production and retention/concentration are responsible for the noteworthy concentration of life at marine fronts. On the other hand, terrestrial landscapes may be modified by human actions, usually increasing contrasts across boundaries (habitat fragmentation); this is another significant difference with marine fronts, which cannot be created by human actions, though they may present significant long-term variability in response to climate change. Fronts and ecotones can represent unique habitats optimal to some species and inhospitable to others; they are places of tension where evolution forces may be at work. The age and history of fronts and ecotones may determine their functional properties, their ecological impacts increasing with their persistence.