The importance of marine subsidies for terrestrial consumers in coastal Peru

The Peruvian coast is one the best examples of cross-ecosystem food web exchanges, in which resources from one of the richest marine ecosystems subsidize consumers in one of the driest deserts on Earth. Marine subsidies are resources that originate in the marine ecosystem, and that contribute to increase the density of consumers in the recipient ecosystem. I examined the effects of marine subsidies on animal populations in the Peruvian coastal desert. ^ I combined several approaches to study the linkages between marine resources and terrestrial consumers, such as surveying the spatial distribution and estimating the relative abundance of terrestrial consumers, studying the diet of geckos and lizards through stomach content analyses, and examining the desert food web with carbon and nitrogen stable isotope analyses. ^ I found that the distribution and diet of desert consumers were tightly coupled to the availability of marine subsidies. I revealed linkages along two pathways of nutrient fluxes: tidal action that washes ashore macroalgae and cadavers of marine organisms, and animal transport in places where pinnipeds and seabirds congregate for reproduction. In the first pathway, intertidal algivivores made marine resources available to terrestrial consumers by moving between the intertidal and supratidal zone. The relative contribution of terrestrial and algal carbon sources varied among terrestrial consumers, because scorpions assimilated a lower proportion of energy from macroalgae than did geckos and solifuges. In the second pathway, I found that pinniped colonies influenced the diet of desert consumers, and contributed to support large populations of lizards and geckos. By combining field observations, and stomach and stable isotope analyses, I constructed a simplified food web for a large sea lion colony, showing the number of trophic levels that originate from pinniped-derived nutrients. ^ My study demonstrates the enormous importance of marine resources for the diet of desert consumers. The near absence of rainfall along the Peruvian coast promotes an extreme dependence of terrestrial consumers on marine resources, and causes permanent food web effects that are affected by temporal variability in marine productivity, rather then temporal patterns of desert plant growth. ^

Stable isotopes reveal complex changes in trophic relationships following nutrient addition in a coastal marine ecosystem

Complex links between the top-down and bottomup forces that structure communities can be disrupted by anthropogenic alterations of natural habitats.We used relative abundance and stable isotopes to examine changes in epifaunal food webs in seagrass (Thalassia testudinum) beds following 6 months of experimental nutrient addition at two sites in Florida Bay (USA) with different ambient fertility. At a eutrophic site, nutrient addition did not strongly affect food web structure, but at a nutrient-poor site, enrichment increased the abundances of crustacean epiphyte grazers, and the diets of these grazers became more varied. Benthic grazers did not change in abundance but shifted their diet away from green macroalgae + associated epiphytes and towards an opportunistic seagrass (Halodule wrightii) that occurred only in nutrient addition treatments. Benthic predators did not change in abundance, but their diets were more varied in enriched plots. Food chain length was short and unaffected by site or nutrient treatment, but increased food web complexity in enriched plots was suggested by increasingly mixed diets. Strong bottom-up modifications of food web structure in the nutrient-limited site and the limited top-down influences of grazers on seagrass epiphyte biomass suggest that, in this system, the bottom-up role of nutrient enrichment can have substantial impacts on community structure, trophic relationships, and, ultimately, the productivity values of the ecosystem.

Spatiotemporal variation of the abundance of calcareous green macroalgae in the Florida Keys: A study of synchrony within a macroalgal functional-form group

The abundance of calcareous green algae was recorded quarterly at 28 sites within the Florida Keys National Marine Sanctuary (FKNMS) for a period of 7 years as part of a sea grass monitoring program. To evaluate the validity of using the functional-form group approach, we designed a sampling method that included the functional-form group and the component genera. This strategy enabled us to analyze the spatiotemporal patterns in the abundance of calcareous green algae as a group and to describe synchronous behavior among its genera through the application of a nonlinear regression model to both categories of data. Spatial analyses revealed that, in general, all genera displayed long-term trends of increasing abundance at most sites; however, at some sites the long-term trends for genera opposed one another. Strong synchrony in the timing of seasonal changes was found among all genera, possibly reflecting similar reproductive and seasonal growth pattern, but the variability in the magnitude of seasonal changes was very high among genera and sites. No spatial patterns were found in long-term or seasonal changes; the only significant relation detected was for slope, with sites closer to land showing higher values, suggesting that some factors associated with land proximity are affecting this increase. We conclude that the abundances of genera behaved differently from the functional-form group, indicating that the use of the functionalform group approach may be unsuitable to detect changes in sea grass community structure in the FKNMS at the existing temporal and spatial scale of the monitoring program.

Tropical seagrass-associated macroalgae distributions and trends relative to water quality

Tropical coastal marine ecosystems including mangroves, seagrass beds and coral reef communities are undergoing intense degradation in response to natural and human disturbances, therefore, understanding the causes and mechanisms present challenges for scientist and managers. In order to protect our marine resources, determining the effects of nutrient loads on these coastal systems has become a key management goal. Data from monitoring programs were used to detect trends of macroalgae abundances and develop correlations with nutrient availability, as well as forecast potential responses of the communities monitored. Using eight years of data (1996–2003) from complementary but independent monitoring programs in seagrass beds and water quality of the Florida Keys National Marine Sanctuary (FKNMS), we: (1) described the distribution and abundance of macroalgae groups; (2) analyzed the status and spatiotemporal trends of macroalgae groups; and (3) explored the connection between water quality and the macroalgae distribution in the FKNMS. In the seagrass beds of the FKNMS calcareous green algae were the dominant macroalgae group followed by the red group; brown and calcareous red algae were present but in lower abundance. Spatiotemporal patterns of the macroalgae groups were analyzed with a non-linear regression model of the abundance data. For the period of record, all macroalgae groups increased in abundance (Abi) at most sites, with calcareous green algae increasing the most. Calcareous green algae and red algae exhibited seasonal pattern with peak abundances (Φi) mainly in summer for calcareous green and mainly in winter for red. Macroalgae Abi and long-term trend (mi) were correlated in a distinctive way with water quality parameters. Both the Abi and mi of calcareous green algae had positive correlations with NO3−, NO2−, total nitrogen (TN) and total organic carbon (TOC). Red algae Abi had a positive correlation with NO2−, TN, total phosphorus and TOC, and the mi in red algae was positively correlated with N:P. In contrast brown and calcareous red algae Abi had negative correlations with N:P. These results suggest that calcareous green algae and red algae are responding mainly to increases in N availability, a process that is happening in inshore sites. A combination of spatially variable factors such as local current patterns, nutrient sources, and habitat characteristics result in a complex array of the macroalgae community in the seagrass beds of the FKNMS.

Spatio-temporal patterns and nutrient status of macroalgae in a heavily managed region of Biscayne Bay, Florida, USA

The coastal bays of South Florida are located downstream of the Florida Everglades, where a comprehensive restoration plan will strongly impact the hydrology of the region. Submerged aquatic vegetation communities are common components of benthic habitats of Biscayne Bay, and will be directly affected by changes in water quality. This study explores community structure, spatio-temporal dynamics, and tissue nutrient content of macroalgae to detect and describe relationships with water quality. The macroalgal community responded to strong variability in salinity; three distinctive macroalgal assemblages were correlated with salinity as follows: (1) low-salinity, dominated by Chara hornemannii and a mix of filamentous algae; (2) brackish, dominated by Penicillus capitatus, Batophora oerstedii, and Acetabularia schenckii; and (3) marine, dominated by Halimeda incrassata and Anadyomene stellata. Tissue-nutrient content was variable in space and time but tissues at all sites had high nitrogen and N:P values, demonstrating high nitrogen availability and phosphorus limitation in this region. This study clearly shows that distinct macroalgal assemblages are related to specific water quality conditions, and that macroalgal assemblages can be used as community-level indicators within an adaptive management framework to evaluate performance and restoration impacts in Biscayne Bay and other regions where both freshwater and nutrient inputs are modified by water management decisions.

Herbivore and nutrient impact on primary producer assemblages in a tropical marine environment

Globally, human populations are increasing and coastal ecosystems are becoming increasingly impacted by anthropogenic stressors. As eutrophication and exploitation of coastal resources increases, primary producer response to these drivers becomes a key indicator of ecosystem stability. Despite the importance of monitoring primary producers such as seagrasses and macroalgae, detailed studies on the response of these benthic habitat components to drivers remain relatively sparse. Utilizing a multi-faceted examination of turtle-seagrass and sea urchin-macroalgae consumer and nutrient dynamics, I elucidate the impact of these drivers in Akumal, Quintana Roo, Mexico. In Yal Ku Lagoon, macroalgae bioindicators signified high nutrient availability, which is important for further studies, but did not consistently follow published trends reflecting decreased δ 15N content with distance from suspected source. In Akumal Bay, eutrophication and grazing by turtles and fishes combine to structure patches within the seagrass beds. Grazed seagrass patches had higher structural complexity and productivity than patches continually grazed by turtles and fishes. Results from this study indicate that patch abandonment may follow giving-up density theory, the first to be recorded in the marine environment. As Diadema antillarum populations recover after their massive mortality thirty years ago, the role these echinoids will have in reducing macroalgae cover and altering ecosystem state remains to be clear. Although Diadema antillarum densities within the coral reef ecosystem were comparable to other regions within the Caribbean, the echinoid population in Akumal Bay was an insufficient driver to prevent dominance of a turf-algal-sediment (TAS) state. After a four year study, declining coral cover coupled with increased algal cover suggests that the TAS-dominated state is likely to persist over time despite echinoid recovery. Studies on macroalgal diversity and nutrients within this same region of echinoids indicated diversity and nutrient content of macroalgae increased, which may further increase the persistence of the algal-dominated state. This study provides valuable insight into the variable effects of herbivores and nutrients on primary producers within a tropical coastal ecosystem. Results from this work challenge many of the currently accepted theories on primary producer response to nutrients and herbivory while providing a framework for further studies into these dynamics.

Macroalgae Decrease Growth and Alter Microbial Community Structure of the Reef-Building Coral, Porites astreoides

With the continued and unprecedented decline of coral reefs worldwide, evaluating the factors that contribute to coral demise is of critical importance. As coral cover declines, macroalgae are becoming more common on tropical reefs. Interactions between these macroalgae and corals may alter the coral microbiome, which is thought to play an important role in colony health and survival. Together, such changes in benthic macroalgae and in the coral microbiome may result in a feedback mechanism that contributes to additional coral cover loss. To determine if macroalgae alter the coral microbiome, we conducted a field-based experiment in which the coral Porites astreoides was placed in competition with five species of macroalgae. Macroalgal contact increased variance in the coral-associated microbial community, and two algal species significantly altered microbial community composition. All macroalgae caused the disappearance of a γ-proteobacterium previously hypothesized to be an important mutualist of P. astreoides. Macroalgal contact also triggered: 1) increases or 2) decreases in microbial taxa already present in corals, 3) establishment of new taxa to the coral microbiome, and 4) vectoring and growth of microbial taxa from the macroalgae to the coral. Furthermore, macroalgal competition decreased coral growth rates by an average of 36.8%. Overall, this study found that competition between corals and certain species of macroalgae leads to an altered coral microbiome, providing a potential mechanism by which macroalgae-coral interactions reduce coral health and lead to coral loss on impacted reefs.

Stable Isotope Biogeochemistry of South Florida Coastal Marine Ecosystems

Southeast Florida’s continual urban expansion will potentially increase anthropogenic pollution in adjacent coastal marine systems. Furthermore, increased nutrient loads could have detrimental effects on the already threatened Florida Reef Tract. The present study uses a stable isotopic approach to determine the sources and the impact of nutrients on the Florida Reef Tract. δ13C and δ15N analysis of macroalgae, sponges, and sediment were analyzed in order to determine nutrient inputs in this region. While δ13C data did not display any significant trends spatially, δ15N values of the majority of biota exhibited a strong East to West gradient with more enriched values close to shore. Relative enrichment in δ15N values were measured for sediments sampled along the Florida Reef Tract in comparison to a pristine Marquesas Keys sediment core. The δ15N data also implies that shoreline anthropogenic nutrients have more nutrient loading implications on reefs than major point sources.

The importance of marine subsidies for terrestrial consumers in coastal Peru

The Peruvian coast is one the best examples of cross-ecosystem food web exchanges, in which resources from one of the richest marine ecosystems subsidize consumers in one of the driest deserts on Earth. Marine subsidies are resources that originate in the marine ecosystem, and that contribute to increase the density of consumers in the recipient ecosystem. I examined the effects of marine subsidies on animal populations in the Peruvian coastal desert. I combined several approaches to study the linkages between marine resources and terrestrial consumers, such as surveying the spatial distribution and estimating the relative abundance of terrestrial consumers, studying the diet of geckos and lizards through stomach content analyses, and examining the desert food web with carbon and nitrogen stable isotope analyses. I found that the distribution and diet of desert consumers were tightly coupled to the availability of marine subsidies. I revealed linkages along two pathways of nutrient fluxes: tidal action that washes ashore macroalgae and cadavers of marine organisms, and animal transport in places where pinnipeds and seabirds congregate for reproduction. In the first pathway, intertidal algivivores made marine resources available to terrestrial consumers by moving between the intertidal and supratidal zone. The relative contribution of terrestrial and algal carbon sources varied among terrestrial consumers, because scorpions assimilated a lower proportion of energy from macroalgae than did geckos and solifuges. In the second pathway, I found that pinniped colonies influenced the diet of desert consumers, and contributed to support large populations of lizards and geckos. By combining field observations, and stomach and stable isotope analyses, I constructed a simplified food web for a large sea lion colony, showing the number of trophic levels that originate from pinniped-derived nutrients. My study demonstrates the enormous importance of marine resources for the diet of desert consumers. The near absence of rainfall along the Peruvian coast promotes an extreme dependence of terrestrial consumers on marine resources, and causes permanent food web effects that are affected by temporal variability in marine productivity, rather then temporal patterns of desert plant growth.