"You find quite a different food chain 

 than the one that's normally 

 present." — Hans Paerl 



algal bloom excludes 95 percent of all 

 the other algae in the water. That 

 could mean bad news for the food 

 chain. Paerl has found that blue-green 

 algae are not a desirable food source 

 and that the blooms are probably 

 altering the food chain. 



Zooplankton, a major food source 

 for many fish and shellfish, feed on 

 phytoplankton, the base of the food 

 chain. But as soon as the blue-greens 

 begin to take over, the zooplankters 

 disappear from the water column. It 

 may be that the blooms form particles 

 too large for the zooplankton to eat or 

 that the algae are toxic to the zoo- 

 plankton. 



Whatever the case, Paerl says the 

 food chain undergoes major changes 

 under bloom conditions. "You find 

 quite a different food chain than the 

 one that's normally present," he says. 

 "Our experiments indicate there's not 

 efficient transfer of this material into 

 the food chain." 



Since the larger zooplankton won't 

 eat the algae, smaller organisms must 

 consume the green scum. The smaller 

 organisms may not be desirable food 

 sources for fish in the food chain, says 

 Paerl. 



It also could mean a change in the 

 fish community from plankton-feed- 

 ers to bottom-dwellers. Paerl says, for 

 example, catfish may survive the 

 blooms on the Chowan while the 

 striped bass population could suffer. 



Paerl says there's such a supply of 

 nutrients constantly being recycled in 



Photo by Nancy Davis 



the Neuse that, when conditions are 

 right, a bloom can flourish for months 

 without using up the supply. 



Usually, it's a change in the weather 

 or in the salinity of the water that kills 

 a bloom. Paerl says blue-green algae 

 can't tolerate even low levels of 

 salinity. He's found that as river flow 

 slows, the fresh water moves toward 

 the salt water in the estuary at a 

 slower rate. At the same time, the 

 heavier salt water moves along the 

 bottom in a "wedge," until it even- 

 tually mixes with the rest of the water 

 column. When the salt wedge meets 

 the bloom, the algae die and sink down 

 the water column. 



Once the algae are dead, the troubles 

 are only just beginning, says Paerl. 

 The oxygen demand from the decom- 

 position process lowers the oxygen 

 level in the water. This year, when the 

 salt wedge moved into the bloom area, 

 the oxygen level went from five 

 milligrams per liter to less than one 

 milligram per liter in one week — a 

 drastic change, says Paerl. According 

 to the state water quality standards, 

 five milligrams per liter are necessary 

 to support a variety of fish life. 



Although there were no big fish kills 

 on the Neuse as a result of that bloom, 

 biologists say low oxygen levels often 

 cause kills. 



Stanley and Christian have found 

 that the blooms upriver may cause 

 even more changes in the estuary. Af- 

 ter the bloom dies, its breakdown 

 products are probably swept down- 



stream into the estuary, says Chris- 

 tian. There they may decompose or 

 settle to the bottom and become part 

 of the sediments. 



Stanley and Christian say that all 

 that dead algae affect the nitrogen- 

 cycling, or where the nitrogen atoms 

 are going in the water, in the lower 

 Neuse and in the estuary. 



Under non-bloom conditions, there 

 is a lot of inorganic nitrogen available 

 in the estuary. But Stanley and Chris- 

 tian have found that, under bloom 

 conditions, the algae use up that in- 

 organic nitrogen and release organic 

 nitrogen. That could mean that 

 organisms that use inorganic nitrogen 

 in the estuary may not be finding all 

 they need. 



One of the problems in studying an 

 algal bloom is that the bloom lasts only 

 a few months. Researchers are forced 

 to take as many samples as possible 

 during the bloom and retreat to the 

 laboratory during the winter months 

 to find answers to their questions. 



Stanley and Christian have freezers 

 full of blue-green algae they've col- 

 lected from this year's bloom. By the 

 time the next bloom is choking the 

 Neuse, they may be able to answer 

 questions like, where the algae go when 

 they die, how the algae release 

 nutrients in the decomposition process, 

 and what effects the algae from up- 

 stream will have on the estuary. 



— Nancy Davis 



Paerl uses hydrocorrals like these to trace the fate of algal blooms 



