Robert A. Johnson

University of Wisconsin–Madison | UW · Department of Integrative Biology

Ecosystem “hot spots” and “hot moments”—respectively, places and times of disproportionately high biogeochemical activity—are an important and often invoked concept in ecosystem science. Despite the popularity of the concept, there is no standard approach to quantifying hot spots and hot moments, hindering progress in understanding the phenomenon....

Concerns over the sustainability of green turtle grazing have become a central topic in discussions of seagrass protection. Understanding grazing plot dynamics and aging is critical to evaluating the sustainability of grazing and understanding the role green turtles play in structuring seagrass foraging habitats. We investigated grazing plot dynami...

Populations of green turtles (Chelonia mydas), a megaherbivore that consumes seagrasses via cultivation grazing, are recovering worldwide. Information on plant-mediated effects on herbivore foraging behavior is critical to understanding plant-herbivore interactions and sustainability of grazing as ecosystems continue to change. In a Caribbean seagr...

Seagrasses form productive marine ecosystems that serve as important foraging grounds for grazers. Meadow productivity is vulnerable to environmental change, however, because environmental factors often strongly regulate seagrass growth. Understanding effects of grazing and environmental driver interactions on growth dynamics is therefore needed to...

Recovery of green turtles (Chelonia mydas), mega-herbivores that consume seagrasses, is resulting in dramatic ecosystem-wide changes as meadows are returned to a natural grazed state. The green turtle grazing strategy, with long-term cultivation of meadows and high foraging site fidelity, is distinct from other terrestrial and aquatic mega-herbivor...

What happens in meadows after populations of natural grazers rebound following centuries of low abundance? Many seagrass ecosystems are now experiencing this phenomenon with the recovery of green turtles (Chelonia mydas), large‐bodied marine herbivores that feed on seagrasses. These seagrass ecosystems provide a rare opportunity to study ecosystem‐...

Seagrass meadows host diverse invertebrate faunal communities. Infaunal organisms residing in the sediments of meadows play important roles in the functioning of these ecosystems, such as the breakdown of organic matter. Disturbance to the benthic environment through grazing by megaherbivores, such as dugongs (Dugong dugon), can reduce infauna abun...

Seagrass meadows buffer sediments against resuspension and erosion by reducing water velocity and attenuating wave energy, thereby promoting accumulation of sediment and associated carbon. Grazing by green turtles (Chelonia mydas) can significantly reduce the aboveground canopy in meadows. Increasing green turtle population sizes will return more s...

Increasing green turtle abundance will lead to increased grazing within seagrass habitats—ecosystems that are important for carbon sequestration and storage. However, it is not well understood how carbon dynamics in these ecosystems respond to grazing and whether a response differs among meadows or locations. We measured seagrass ecosystem metaboli...

Seagrass meadows are often comprised of diverse assemblages of seagrasses and algae. Green turtles (Chelonia mydas) are a prominent megaherbivore in seagrass meadows—capable of consuming large amounts of aboveground seagrass biomass and driving shifts in seagrass species dominance. Previous green turtle grazing studies have focused on diversity and...

Seagrass meadows are important sites for carbon storage. Green turtles (Chelonia mydas) are marine megaherbivores that consume seagrass throughout much of their global range. With successful conservation efforts, turtle abundance will increase, leading to more meadows being returned to their natural grazed state. There is concern this may lead to a...

Theory suggests that alternative resources may begin to support a food web when highly used resources become less available relative to alternatives. To test the potential for alternative resources to support consumers, we experimentally darkened a lake whose consumers had relied heavily on algal resources (phytoplankton and benthic algae). We esti...

Many lakes have positive, heterograde vertical oxygen (O2) profiles with a metalimnetic maximum usually assumed to be the result of biological O2 production. However, supersaturated metalimnetic O2 maxima are formed by biological processes (net photosynthetic production of O2) and physical processes (warming of gasses trapped below the thermocline)...

Somatic growth rates are variable within and among aggregations of sea turtles. On the north coast of the island of Great Inagua in the southern Bahamas, growth rates of immature green turtles (Chelonia mydas) spanning the post-recruitment development phase have been measured for over 30 years in a capture – mark – recapture study. Much of the vari...

Significance Large changes can occur when ecosystems cross certain thresholds. Crossing such thresholds poses a challenge to ecosystem management because the positions of the threshold are uncertain and change over time. However, as an ecosystem approaches a threshold its resilience declines, resulting in changes in system dynamics that increase va...

Background/Question/Methods A large portion of the total organic matter in many lakes is terrestrial in origin, and lake consumers can be supported by both terrestrial (allochthonous) and aquatic (autochthonous) resources. With the progression of global climate change, freshwaters may receive higher loadings of allochthonous organic matter, which...

Background/Question/Methods Regime shifts are rapid and dramatic transitions to alternate ecosystem states. While regime shifts are difficult to predict, statistical indicators may provide early warnings. Specifically as critical thresholds are approached, the variance and autocorrelation of time series often indicate “critical slowing down” (i.e...

A Bayesian mixing model and stable isotopes of carbon, nitrogen, and hydrogen were used to evaluate the extent to which six consumers (three fishes, two zooplankton, and a snail) in a naturally productive lake used terrestrial resources, epilimnetic and metalimnetic phytoplankton, benthic algae, and macrophytes. Resource use varied with consumer ha...

Some aquatic systems have disproportionately high nutrient processing rates, and may be important to nutrient retention within river networks. However, the contribution of such biogeochemical hot spots also depends on water residence time and hydrologic connections within the system. We examined the balance of these factors in a comparative study o...