In this study, calibrated watershed and reservoir models are used to explore a range of possible watershed conditions and potential management options to reduce available nutrients and algal growth in the Lake Waco reservoir. The management options are divided between watershed and reservoir options. The watershed management options include wetland construction, manure haul-off, agriculture conversion to pasture, absolute nutrient retention in the watershed and control of urban nutrient run-off. For the reservoir, management options of phosphorus inactivation and increased algal consumption by grazers were evaluated. For all individual management scenarios, only complete conversion of agricultural lands into rangeland decreased nutrient levels and algae growth significantly and achieved target levels for chlorophyll-a and total phosphorus. Combined management scenarios including wetland construction, manure haul-off from dairy operations and increased in-reservoir herbivory could further reduce chlorophyll-a and nutrient values, but with less efficiency than agricultural conversion alone. The management option study showed that decreasing nutrient inputs and water clarity were important factors for controlling algal growth in Lake Waco, and that substantial reduction in total phosphorus is needed to achieve target conditions.
Change was in the air The atmospheric fraction of molecular oxygen gas, O 2 , currently at 21%, is thought to have varied between around 35 and 15% over the past 500 million years. Because O 2 is not a greenhouse gas, often this variability has not been considered in studies of climate change. Poulson and Wright show that indirect effects of oxygen abundance, caused by contributions to atmospheric pressure and mean molecular weight, can affect precipitation and atmospheric humidity (see the Perspective by Peppe and Royer). These effects may thus have produced significant changes in the strength of greenhouse forcing by water vapor, surface air temperatures, and the hydrological cycle in the geological past. Science , this issue p. 1238 ; see also p. 1210
Significance Computer-assisted studies of natural history that consider extinct plant function contribute to the understanding of how Paleozoic glacial cycles controlled the distribution of forest cover and continental surface erosion. Simulated plant water balance supports widespread vegetation during the late Paleozoic ice age (LPIA). However, physiological inference suggests that plant freezing limited the geographic distribution of vegetation. Assuming LPIA plants had limited freeze tolerance, we found that increased surface runoff could have contributed to late Paleozoic climate change. Modeling that combines deep time climate reconstructions and paleobotanical data improves understanding of past Earth systems, which can help project future change.
Ecosystem process models provide unique insight into terrestrial ecosystems by employing a modern understanding of ecophysiological processes within a dynamic environmental framework. We apply this framework to deep-time ecosystems made up of extinct plants by constructing plant functional types using fossil remains and simulating—as close as possible—the in vivo response of extinct taxa to their paleoclimatic environment. To accomplish this, foliar characteristics including maximum stomatal conductance, distance from leaf vein to stomata, and cuticular carbon and nitrogen were input as model parameters derived from measurements of well-preserved Pennsylvanian-age fossil leaves. With these inputs, we modeled a terrestrial tropical forest ecosystem dominated by "iconic" plant types of the Pennsylvanian (∼323–299 Ma) including arborescent lycopsids, medullosans, cordaitaleans, and tree ferns using a modified version of the process model BIOME-BGC, which we refer to as Paleo-BGC. Modeled carbon and water—and, for the first time, nitrogen—budgets of a tropical ecosystem from Euramerica driven by daily meteorology are simulated using the Global Circulation Model GENESIS 3.0. Key findings are: lycopsids have the lowest daily leaf water potential, soil water content, surface runoff, and degree of nitrogen leaching indicating an intensive water use strategy compared to medullosans, cordaitaleans, and tree ferns that have increasingly lower simulated water use, greater surface, and nitrogen loss in this order; modeled vegetation response to aridification, which was caused by reduced precipitation and intensified through the close of the Carboniferous and into the Permian shows that lycopsids and medullosans have the lowest tolerance for precipitation decrease compared to cordaitaleans and tree ferns, consistent with the paleobotanical record of occurrence of floral turnovers through the Middle Pennsylvanian through earliest Permian; elevated atmospheric pO2, hypothesized as characteristic for the latter half of the Pennsylvanian and early Permian (∼299–272 Ma), caused higher atmospheric pressure reducing plant transpiration, higher surface water runoff rates, and increased nitrogen export for all plant types simulated, manifested most strongly in the lycopsid dominated ecosystems—with overall only a small reduction in net daily assimilation (≈1 μmol CO2 m−2 s−1). Both aridification and elevated atmospheric oxygen reduced transpiration, increased water retention in soils, with higher surface runoff. With more discharge, enhanced and higher short-term surface soil loss and silicate weathering would have been possible in broad regions of the paleotropics during the late Carboniferous and early Permian. These results are only obtainable by integrating multiple, fossil-derived measurements into the simulation framework of an ecosystem model that utilizes daily meteorology.
Goldblatt argues that a decrease in pressure broadening of absorption lines in an atmosphere with low oxygen leads to an increase in outgoing longwave radiation and atmospheric cooling. We demonstrate that cloud and water vapor feedbacks in a global climate model compensate for these decreases and lead to atmospheric warming.
Scanning electron microscopy (SEM) is widely used to investigate the surface morphology, and physiological state of plant leaves. Conventionally used methods for sample preparation are invasive, irreversible, require skill and expensive equipment, and are time and labor consuming. This study demonstrates a method to obtain in vivo surface information of plant leaves by imaging replicas with SEM that is rapid and non-invasive. Dental putty was applied to the leaves for 5 minutes and then removed. Replicas were then imaged with SEM and compared to fresh leaves, and leaves that were processed conventionally by chemical fixation, dehydration and critical point drying. The surface structure of leaves was well preserved on the replicas. The outline of epidermal as well as guard cells could be clearly distinguished enabling determination of stomatal density. Comparison of the dimensions of guard cells revealed that replicas did not differ from fresh leaves, while conventional sample preparation induced strong shrinkage (-40% in length and -38% in width) of the cells when compared to guard cells on fresh leaves. Tilting the replicas enabled clear measurement of stomatal aperture dimensions. Summing up, the major advantages of this method are that it is inexpensive, non-toxic, simple to apply, can be performed in the field, and that results on stomatal density and in vivo stomatal dimensions in 3D can be obtained in a few minutes.
Abstract The Cretaceous is characterized as a greenhouse climate from elevated atmospheric carbon dioxide concentrations, transgressive seas, and temperate ecosystems at polar paleolatitudes. Here we test the hypothesis that the early Cretaceous was a cold climate state with a new Aptian atmospheric carbon dioxide record from the C 3 plant proxy and early Cretaceous sea level curve from stable oxygen isotopes of belemnites and benthic foraminifera. Results show that carbon dioxide concentrations were persistently below 840 ppm during the Aptian, validating recent General Circulation Model simulations of ice sheets on Antarctica at those concentrations. In addition, sea level was estimated to be within the ice sheet window for much of the early Cretaceous prior to the Albian. This background state appears to have been episodically interrupted by Large Igneous Province volcanism followed by long-term carbon burial from weathering. We hypothesize that the early Cretaceous was largely an icehouse punctuated by warm snaps.
