Top-down and bottom-up regulation in a high nutrient–high herbivory coastal ecosystem (original) (raw)
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Proceedings of the Estonian Academy of Sciences, 2014
Marine macroalgal communities are among the most productive habitats worldwide. They provide energy and matter to higher trophic levels and support other important functions for ecosystems and services for human society. To date it is not clear to what extent irradiance, nutrient loading, and mesoherbivores regulate the primary productivity of a community. In a factorial field experiment we evaluated the interactive effect of short-term pulses of elevated nutrients and of the activity of grazers on the photosynthesis (in terms of the rate of oxygen production per unit mass) of communities dominated by the perennial Fucus vesiculosus and the ephemeral Cladophora glomerata in the northern Baltic Sea. This experimental manipulation had no effect on the community dominated by F. vesiculosus. A 12-hour addition of herbivores decreased the photosynthetic production of the macroalgal community dominated by C. glomerata in spring but increased its production in summer. The simultaneous addition of nutrients and herbivores in summer reversed the effect. A 4-times longer manipulation had no effect on the C. glomerata production. Differences in the responses between separate and interactive effects imply that the photosynthetic production of a community cannot be predicted by separate effects of the same variables. Our experiment also indicated that macroalgal communities dominated by F. vesiculosus covered by epiphytic macrophytes performed stably under different stress regimes and could buffer moderate short-term disturbances due to elevated nutrient loads and/or herbivory of either natural or human origin.
Estuaries, 1998
Macroalgal biomass and competitive interactions among primary producers in coastal ecosystems may be conU'olled by bottom-up processes such as nutrient supply and top-down processes such as grazing, as well as other environmental factors. To determine the relative importance of bottom-up and top-down processes under different nutrient loading conditions, we estimated potential amphipod and isopod grazer impact on a dominant macroalgal species in three estuaries in Waquoit Bay, Cape Cod, Massachusetts, that are subject to different nitrogen loading rates. We calculated growth increases and grazing losses in each estuary based on monthly benthic survey data of macrophyte biomass and herbivore abundance, field grazing rates of amphipods (Microdeutopus gryllotalpa and Cymadusa compta) and an isopod (Idotea baltica) on the preferred and most abundant macroalga (Cladophora vagabunda) and laboratory grazing rates for the remaining species, and in situ macroalgal growth rates. As nitrogen loading rates increased, macroalgal biomass increased (3x), eelgrass (Zostera marina) was lost, and herbivore abundance decreased (aAx). Grazing rates increased with relative size of grazer (I. baltica > C. compta > M. gry//otodpa) and, for two of the three species investigated, were faster on algae from the high-nitrogen estuary in comparison to the low-nitrogen estuary, paralleling the increased macroalgal tissue percent nitrogen with nitrogen load. Macroalgal growth rates increased (2• with increasing nitrogen loading rate. The comparison between estimated growth increases versus losses of C. vagabunda biomass to grazing suggested first, that grazers could lower macroalgal biomass in midsummer, but only in estuaries subject to lower niUrogen loads. Second, the impact of grazing decreased as nitrogen loading rate increased as a result of the increased macroalgal growth rates and biomass, plus the diminished abundance of grazers. This study suggests the relative impact of top-down and bottom-up controls on primary producers varies depending on rate of nitrogen loading, and specifically, that the impact of herbivory on macroalgal biomass decreases with increasing nitrogen load to estuaries.
Estuaries and Coasts, 2012
Understanding whether nutrient availability and grazing by consumers can control macroalgal growth is important to mitigate blooms. To assess the effect of longterm nitrogen loading on macroalgae, we ran a field experiment in which we measured growth of green and red macroalgae in estuaries where loads and eutrophication status differed. The relative abundances of consumers differed among estuaries with more grazers in non-eutrophied estuaries, an important interaction of bottom-up and topdown controls. In the estuary with the lowest nitrogen load, grazers controlled green macroalgal growth, but in higher nitrogen-loaded estuaries, where grazing was lower, growth of green macroalgae overwhelmed potential grazer control. The red macroalga was not controlled by grazers, even in the estuary where grazing pressure was highest. In the lowloaded estuary, invertebrate predators exerted top-down control over grazers, but predation effects did not cascade to macroalgae. Bottom-up mechanisms dominated control of macroalgae through an interaction of direct stimulation of growth and indirect alteration of consumer abundances, and thus, long-term nutrient regimes are likely determining potential for bloom formation in Waquoit Bay.
Estuarine Coastal and Shelf Science, 2021
Nutrient input drive macroalgal blooms and increases in photosynthetic activity in coastal ecosystems. An intense macroalgal photosynthetic activity can increase the surrounding pH and it could prevent the acidification that often follows an eutrophication process. We tested this hypothesis with field sampling and experiments in a macrotidal (up to 9 m in amplitude) coastal system within a semi-desert region with contrasting eutrophic conditions and Ulva lactuca blooms in the northern Argentinean Patagonia (San Antonio Bay). Our results indicate that daily pH variability during low tide could be controlled by the photosynthetic activity of Ulva lactuca under eutrophic conditions. At seasonal scale, the pH variations were related to environmental features, particularly seawater temperature. Both environmental (i.e. high solar radiation, negligible freshwater inputs and large tidal action) and anthropogenic nutrient inputs into the studied area promote the Ulva lactuca blooms, which in turn increases the surrounding pH in well oxygenated seawater through the intense photosynthetic activity. Our study shows that eutrophication instead of being a driver of acidification, could contribute to its prevention in well oxygenated marine coastal systems located within semi-desert regions.
Importance of benthic nutrient regeneration during initiation of macroalgal blooms in shallow bays
Marine Ecology Progress Series, 2003
Fast growing ephemeral algae are increasingly observed in shallow coastal waters worldwide. This is generally considered a symptom of coastal eutrophication. It has been suggested that the reoccurrence of macroalgal mats, despite ongoing efforts to decrease nutrient loadings, may be explained by the fact that sheltered bays function as self-regenerating systems through benthic regeneration of nutrients. The importance of benthic nutrient regeneration during the initiation of the growth of filamentous green algal mats was investigated in 2 shallow-water bays on the west coast of Sweden. Nutrient regeneration was assessed in situ and in the laboratory, using sediment-water light/dark incubations and pore water distribution patterns. Benthic efflux of inorganic nutrients could supply up to 55 to 100% of the estimated nitrogen demand and 30 to 70% of the phosphorus requirements for the initial macroalgal growth from May to June. However, the availability of the pore water nutrient pool was influenced by the amount and functional type of infauna, and by competition from microphytobenthos. Because of the internal nutrient source of the embayments, it is suggested that there may be a significant time lag between a decreased nutrient supply to coastal waters and improved conditions of shallow water embayments.
Macroalgal blooms in shallow estuaries: Controls and ecophysiological and ecosystem consequences
Limnology and Oceanography, 1997
Macroalgal blooms arc produced by nutrient enrichment of estuaries in which the sea floor lies within the photic zone. We review fcaturcs of macroalgal blooms pointed out in recent literature and summarize work done in the Waquoit Bay Land Margin Ecosystems Research project which suggests that nutrient loads, water residcncc times, presence of fringing salt marshes, and grazing affect macroalgal blooms. Increases in nitrogen supply raise macroalgal N uptake rates, N contents of tissues, photosynthesis-irradiance curves and P,,,.,, and accelerate growth of fronds. The resulting increase in macroalgal biomass is the macroalgal bloom, which can displace other estuarine producers, Fringing marshes and brief water residence impair the intensity of macroalgal blooms. Grazing pressure may control blooms of palatable macroalgac, but only at lower N loading rates. Macroalgal blooms end when growth of the phytoplankton attenuates irradiation reaching the bottom. In cstuarics with brief water rcsidencc times, phytoplankton may not have enough time to grow and shade macrophytcs. High phytoplankton division rates achieved at high nutrient concentrations may compensate for the brief time to divide before cells arc transported out of the estuary. Increased N loads and associated macroalgal blooms pervasively and fundamentally alter estuarinc ecosystems. Macroalgae intercept nutrients regenerated from sediments and thus uncoupIe biogeochemical sedimentary cycles from those in the water column. Macroalgae take up so much N that water quality seen:? high even where N loads are high. Macroalgal C moves more readily through microbial and consumer food webs than C derived from seagrasscs that were replaced by macroalgae. Macroalgae dominate 0, profiles of the water columns of shallow estuaries and thus alter the biogeochemistry of the sediments. Marc frequent hypoxia and habitat changes associated with macroalgal blooms also changes the abundance of bcnthic fauna in affected estuaries. Approaches to rcmediation of the many pervasive cffccts of macroalgal blooms riced to include interception of nutrients at their watcrshcd sources and perhaps removal by harvest of macroalgae or by increased flushing. Although we have much knowledge of macroalgal dynamics, all such management initiatives will require additional information.