experiments are unrealistically high. Gordon and Prouse () did use experimental concentrations that realistically might be found in polluted coastal areas or near spills. They found that, at concentrations of petroleum hydrocarbons in solution below 10 to 30 parts per billion (ppb), photosynthesis was stimulated. Concentrations of the crude and refined oils tested of from 60 to 200 ppb suppressed photosynthesis to varying degrees. Number 2 fuel oil had the greatest effect and depressed photosynthesis by 60 percent at concentrations between 100 and 200 ppb. These results indicate that there is a possibility of both stimulation and inhibition of photosynthesis in areas subject to chronic oil pollution or in the vicinity of a heavy oil spill.

The larvae or young of many benthic and fish species spend time as members of the zooplankton. They are often much more susceptible to toxicants than adults. Larvae of the intertidal barnacle (Elminius modestus) were shown to be killed by 100 parts per million (ppm) of fresh crude oil (). Crude oils have also been shown to be toxic to the planktonic eggs and larvae of some fishes, including cod and herring ().

In addition to potentially acute effects of oil spills on planktonic organisms, there has been concern about the long-term effects of floating oil and tar lumps, which have become alarmingly common on the high seas. If the concentrations of petroleum hydrocarbons in ocean surface waters are being increased by shipping discharges or atmospheric input, there would certainly be concern for the near-surface plankton so important to oceanic productivity. Data on hydrocarbon concentrations in seawater are remarkably scant and historical data are nonexistent, thus making it impossible to predict future trends and effects of oil on the oceanic ecosystem.