enzymes are slowly reaching their optimum temperature.

There is an optimum temperature at which the rate of photosynthesis is maximum.
A suitable experiment for GCSE to determine whether incident light intensity affects the photosynthesis rathe in the pondweed elodea.
This activity is a lab where students design an experiment to test the rate of photosynthesis.

The optimum temperature for the process is somewhere ..
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The authorsnoted that in winter, when the river was in peak flow and the watertemperature below that for optimum microbial activity, appreciableamounts of the herbicide would be washed into the estuary.

plant’s enzymes will be working closer to their optimum temperature

The reason for this is, as mentioned previously each species whether plant or human has an optimum temperature at which they function best.
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This variation is due to differences in species sensitivity, chemical structure (esters, salts, or free acid), and formulation of the herbicide. Although the free acid is the physiologically toxic entity, the ester formulations represent a major hazard to fish when used directly as aquatic herbicides (because they are more readily taken up by fish). Amine salt formulations used to control aquatic weeds do not affect adult fish. The NOEL varies with the species and the formulation: <1 mg/litre (coho salmon) to 50 mg/litre (rainbow trout). Fish larvae are the most sensitive life stage but are unlikely to be affected under normal usage of the herbicide. Long-term adverse effects on fish are observed only at concentrations higher than those produced after 2,4-D has been applied at recommended rates. Few studies are related to the effects of environmental variables, such as temperature and water hardness, on 2,4-D toxicity to fish. Higher temperature possibly increases the toxicity.