Sediment date ( AD, year) (a) (c) 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 Sediment date ( AD, year) 2100 1950 1800 1650 1500 1350 1200 1050 900 750 600 450 300 150 −150 0.00 1.00 2.00 3.00 4.00 5.00 TOC preserved per year (mg cm−2 yr−1) 2.5 3.5 4.5 (d) Shannon’s H′ 355 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 25 50 75 100 125 106 diatoms (number cm−2 yr−1) (b) Sediment date ( AD, year) Sediment date ( AD, year) Eutrophication and Oligotrophication 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 Centric/pennate diatom ratio Core 50-E Core R4-45 Core R4-30 Core R4-50 Figure The geological record of cultural eutrophication in Chesapeake Bay, showing major changes since European settlement The data from four sediment cores are graphed by the average date assigned to each sediment sample (depth layer) according to radiocarbon and pollen methods (a) Total organic carbon (TOC, indicating total system productivity) preserved over time (historic record from AD 150 to AD 1990) (b) Diatom cell numbers per year (AD 400–AD 1990) (c) Diatom assemblage diversity calculated as Shannon’s H (AD 400–AD 1990) (d) Centric/ pennate diatom ratios (AD 400–AD 1990) TOC, diatom numbers, and the centric/pennate diatom ratios all showed a significant and abrupt increase following the time of European settlement in the 1700s The total diatom community diversity, in contrast, significantly decreased post1700s, relative to pre-1700s diversity Graphs (a)–(d) were modified from Cooper SR r (1995) Chesapeake Bay watershed historical land use: impact on water quality and diatom communities Ecological Applications 5: 703–723, with permission from American Association for the Advancement of Science eutrophication, small cyanobacteria become abundant; in parts of the Florida Keys, for example, the formerly clear water over coral reef areas has become an opaque emerald green from picoplanktonic cyanobacteria blooms Trace metals such as iron have also been shown to be limiting to phytoplankton growth in some estuarine and marine waters where N and P are at levels that would otherwise be expected to support more algal production As eutrophication progresses, the previously described shift in temperate-zone phytoplankton assemblage structure from certain diatoms to other diatom species, flagellates, and cyanobacteria causes subtle but important, undesirable changes for the food web that can adversely affect secondary production For example, as reviewed by Kilham et al (1997), some diatom species produce high quantities of certain lipids that are essential for zooplankton reproduction Algal species