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The objective of this study was to examine the differences in the biochemical and elemental stoichiometry of a freshwater centric diatom, Stephanodiscus minutulus (Grun.), under various nutrient regimes. Stephanodiscus minutulus was grown at μ or 22% of μ under limitation by silicon, nitrogen, or phosphorus. Cell sizes for nutrient-limited cultures were significantly smaller than the non-limited cell sizes, with N-limited cells being significantly smaller than all other treatments. Compared with the nutrient-replete treatment, both carbohydrates and lipids increased in Si- and P-limited cells, whereas carbohydrates increased but proteins decreased in N-limited cells. All of the growth-limited cells showed an increase of carbohydrate and triglyceride, and a decrease of cell size and polar lipids as a percentage of total lipids. The non-limited cells also had a significantly higher chl a concentration and galactolipids as a percentage of total lipids than any of the limited treatments, and the low-Si and low-P cells had significantly higher values than the low-N cells. The particulate C concentrations showed significant differences between treatments, with the Si- and P-limited treatments being significantly higher than the N- and non-limited treatments. Particulate Si did not show a strong relationship with any of the parameters measured, and it was the only parameter with no differences between treatments. The low-Si cells had a significantly higher P content (about two times more) than any other treatment, presumably owing to the luxury consumption of P, and a correspondingly high phospholipid concentration. The elemental data showed that S. minutulus had a high P demand with low optimum N:P (4) and Si:P (10) ratios and a C:N:P ratio of 109:16:2.3. The particulate C showed a positive relationship with POM (r = 0.93), dry weight (r = 0.88), lipid (r = 0.87) and protein (r = 0.84, all P < 0.0001). Particulate N showed a positive relationship with galactolipids (r = 0.95), protein (r = 0.90), dry weight (r = 0.78), lipid (r = 0.75), and cell volume (r = 0.64, all P < 0.0001). It is evident that nutrient limitation in the freshwater diatom S. minutulus has pronounced effects on its biochemical and elemental stoichiometry.

1. Cell size and volume changed as a function of the type of resource limitation, with nitrogen‐limited cells being smaller and less dense and phosphorus‐limited cells being larger and more dense than non‐limited cells. 2. The major biochemical constituents of the green alga Ankistrodesmus falcatus varied as a function of nitrogen or phosphorus limitation (15% of maximum growth rate) compared to cells growing at their maximum rate. Nitrogen‐limited cells had much lower protein content and phosphorus‐limited cells had higher carbohydrate and lipid contents than cells growing under no limitation. 3. Phosphorus‐limited cells had a higher total lipid content than either nitrogen‐limited or non‐limited cells, but the lipid class composition was similar. 4. The protein : lipid ratio was lowest (0.38) in the nitrogen‐limited cells, intermediate in the phosphorus‐limited cells (0.44) and highest in the non‐limited control cells (1.14).

Dam removal has been proposed as an effective method of river restoration, but few integrative studies have examined ecological responses to the removal of dams. In 1999, we initiated an interdisciplinary study to determine ecological responses to the removal of a 2 m high dam on lower Manatawny Creek in southeastern Pennsylvania. We used an integrative monitoring program to assess the physical, chemical, and biological responses to dam removal. Following removal in 2000, increased sediment transport has led to major changes in channel form in the former impoundment and downstream reaches. Water quality did not change markedly following removal, probably because of the impoundment's short hydraulic residence time (less than two hours at base flow) and infrequent temperature stratification. When the impoundment was converted to a free flowing reach, the composition of the benthic macroinvertebrate and fish assemblages in this portion of Manatawny Creek shifted dramatically from lentic to lotic taxa. Some fish species inhabiting the free flowing reach downstream from the dam were negatively affected by large scale sediment transport and habitat alteration following dam removal, but this appears to be a short term response. Based on our observations and experiences in this study, we provide a list of issues to evaluate when considering future dam removals.