Algae-clay aggregates were formed when algal and clay suspensions were mixed in the presence of an electrolyte. The maximum ratio of clay to algae in the aggregates was 1.7, 0.2, and 0.03 milligrams of clay per milligram of algae (wet weight) for Anabaena, Chlamydomonas, and Chlorella sp., respectively. The aggregates formed at Ca(2+) concentrations higher than 5 x 10(-4)M or Na(+) concentrations higher than 2 x 10(-2). The mutualf flocculation and subsequent sedimentation have many practical and ecological implications for bodies of water.

The transition between phosphorus limitation of primary production in freshwater and nitrogen limitation in seawater was examined along an estuarine gradient simulated in 4 large 13 m" enclosures connected in a series and containing pelagic and benthic subsystems. Nominal salinities of 0, 5, 10 and 25 ppt were maintained by exchanging appropriate volumes of water between enclosures. Klver water, which served as a freshwater endmember, was naturally high in N relative to P, while the oceanic endmember (water from Narragansett Bay, RI, USA) was low in N relative to P. Production in the water column was supported by external inputs and recycled nutrients. Bioassays, inorganic nutrient concentrations and N:P ratios of the seston and inorganic nutrients indicated that phosphorus was limiting at 0, 5 and 10 ppt, while nitrogen was limiting at 25 ppt. Coincident with this shift in limiting nutrient was a shift in the N:P ratio of nutrient supply from greater than the Redfield ratio of 16 to less than 16. External inputs established relative rates of supply in each enclosure. The relative proportion of N and P in external inputs was largely a function of the hydrodynamic mixing of fresh (high N, low P) and salt water (low N, high P) endmembers. At the scale of the estuarine segment or enclosure, neither recycled inputs from the benthos and water column, nitrogen fixation nor internal losses of N and P to sedimentation and/or denitrification materially altered relative supply rates, despite a hydrodynamic residence time of 27 d. KEY WORDS: Estuaries. Primary production. Nutrient limitation. Nitrogen. Phosphorus

The effects of applied fertilizer P and soil‐water stress on the growth and amounts and forms of plant P were investigated to provide further information on the potential of plant material to contribute P to runoff. Cotton [Gossypium hirsutum (L.)], little bluestem (Andropogon scorparius Michx.), sorghum [sorghum sudanense (Piper) Stapf.], and soybean [Glycine max (L.) Merr.], plants were grown in the field on Durant loam, a member of the fine, montmorillonitic, thermic Vertic Argiustolls. Fertilizer P additions of 0, 50, 100, and 200 kg/ha were made and a water‐stress treatment initiated by applying only one‐third the amount of water lost as evapotranspiration. Plant growth and P content were determined at weekly intervals following plant emergence. Growth and total P content increased with an increase in applied fertilizer P for all plants except little bluestem. The total P and inorganic P content of the plants decreased gradually with growth. Inorganic P constituted the major proportion of plant P (approximately 80, 70, 70, and 80% for cotton, little bluestem, sorghum, and soybean, respectively), and remained constant during plant growth. A three‐fold reduction in soil‐water content (60‐fold increase in soil‐water suction) resulted in a decrease in plant growth and total and inorganic P content. The effect of the soil‐water stress was reduced by increases in applied fertilizer P. It was calculated, however, that the addition of fertilizer P was not a viable economic method of increasing crop yields to negate any drop in yield due to drought conditions.

Synopsis Lysine and arginine contents of bromegrass varied inversely with concentration of calcium and potassium in growth medium. Aspartic acid accounted for much of increased nitrogen content of bromegrass when heavily fertilized with nitrogen.