Sediment records of phosphorus-driven shifts to phytoplanktondominance in shallow Florida lakes     

William F. Kenney  Matthew N. Waters  Claire L. Schelske  Mark Brenner 《Journal of Paleolimnology》2002,27(3):367-377

Primary producer community structure (PPCS) in shallow lakes isinfluenced by phosphorus (P) load and water column P concentration.Theoretically PPCS may shift between phytoplankton and macrophyte states withintermediate P loading, but phytoplankton dominate when P loading exceeds acritical threshold. We analyzed sediment cores from five shallow, eutrophiclakes (size range: 0.6 to 125 km²) that arephytoplankton dominated to determine whether the development of the currentstate was associated stratigraphically with an increase in sediment total P(TP) and a shift in PPCS. We used sponge biogenic silica(BSi_Sponges) concentrations and total carbon to total nitrogenratios (TC:TN) as proxies for macrophyte abundance and sediment organic mattersource, respectively. Three stratigraphic groups of sediments were identifiedwith k-means cluster analysis. These samples were grouped by increasing TPconcentrations and decreasing age and identified as macrophyte, transitionaland phytoplankton sediments. Results show that as P loading increased in thelate 19th and early 20th centuries, the lakes producedsediments with an increasing contribution from phytoplankton. Four of our lakesmay represent a subset of shallow lakes because of their large size (30 to 125km²) and relatively rapid historic P enrichment. Inthese Florida lakes, PPCS shifted to phytoplankton dominance with nopaleolimnological record of lake-wide alternating stable states or of lake-widephytoplankton dominance before anthropogenic P enrichment.  相似文献   

J. B. JACKSON AS A CRITIC OF MODERN ARCHITECTURE     

HELEN LEFKOWITZ HOROWITZ 《Geographical review》1998,88(4):465-473

ABSTRACT. Understanding what the American landscape meant to J. B. Jackson requires an exploration of his background, education, and antagonism to the International Style. No full critique of modernism appears in Jackson's mature published work. However, knowledge that the first issues of Landscape magazine in 1951 and 1952 were the work of a single author leads to discovery of Jackson's pseudonyms, especially H. G. West, P. G. Anson, G. A. Feather, and A. W. Conway. This article examines Jackson's pseudonymous writings and links them to his well-known essays on the landscape: “The Westward-Moving House,” “Other-Directed Houses,” and “Southeast to Turkey.”  相似文献