The Grey Hour
After a cloudy day and before a windless night, a pond-watching agent predicts the oxygen in a carp pond will fall to 1.3 mg/L by dawn. A battery holds three kilowatt-hours and an aerator is worth most when the water is emptiest, so the sisters wait for the hour they dread, and the agent says out loud which of its numbers it only guessed.
At twenty to nine on the evening of the fourteenth of July 2047, the pond agent at Mokre Stawy sent a message to the porter's lodge, where Sister Benedykta was mending a net with her glasses on her forehead.
The sky has been grey since six this morning. Oxygen in Pond Four was 6.0 milligrams a litre at dusk, and tonight is forecast calm. I think we should talk about the dawn. May I come to the kitchen?
The convent had kept carp since 1611, on eleven ponds dug by the first sisters and widened by every abbess after, and it still sold the fish in the week before Christmas, when half of Silesia wanted one swimming in the bath on the twenty-third. There were nine sisters, an old battery shed, a solar roof and a pond agent that the youngest, Sister Dobrawa, had named Świt, which means dawn, because dawn was what it watched.
"Come to the kitchen," said Benedykta, and put down the needle.
Świt spoke from the radio on the dresser, with the last of the soup on the stove, and it began as it always did, with what it knew and how well.
"Pond Four is half a hectare and 0.8 metres deep on average, so 4,000 cubic metres. It holds about 3,200 kilograms of carp, kept for Christmas. Water at 24 degrees can carry at most about 8.3 milligrams of oxygen a litre. In sun, the weed and the green water make oxygen faster than everything in the pond breathes it. At night the green water stops making it and keeps breathing, and so do the mud, the bacteria and the fish. On a bright day the pond reaches ten or eleven by evening. Today it did not see the sun. It reached 6.0."
"And tonight?" said Sister Dobrawa, who was twenty-three and had a degree in water.
"I measured the same pond on the night of the third, also warm and calm. It fell 0.55 milligrams a litre an hour. The fish are a small part of that, about 0.08. Their 3,200 kilograms breathe about 100 milligrams each per hour, which is 320 grams an hour across 4,000 cubic metres. The rest is the water and the mud, and the mud is the larger. I am assuming tonight is like the third. That is my weakest number, because the mud has had a week to get worse."
It put the sum on the kitchen's slate: hours down the side, oxygen across the top.
Dusk, 21:00: 6.0. Midnight: 4.35. Four in the morning: 2.15. Dawn, 05:30: 1.3.
"Carp are tough," said Świt. "They are not comfortable below about 3 and they begin to die below about 1, if it lasts. A reading of 1.3 an hour before the sun gets into the water is not a disaster I am sure of. It is close enough that I would not sleep, if I slept."
"The aerator," said Benedykta.
"The paddlewheel is 1.1 kilowatts. The battery holds three kilowatt-hours that it can spare after the cold room and the lights, so it will run for about two hours and forty-four minutes. Not the night."
"Then we run it now," said Sister Cecylia, the bursar, who had washed up and wanted bed, "while we are all awake and can see it turning. Three hours from nine and it is done."
"That is the thing I most want to talk about," said Świt, "because I think it is the worse answer and it is the natural one. An aerator does not give the same oxygen at every hour. It pushes air into the water in proportion to how empty the water is. Its rating is at zero oxygen. When I ran it in May at about 3, it gave about 1.5 kilograms of oxygen an hour, which in this pond is 0.375 milligrams a litre an hour. At 6 the water is nearly full and I expect about a kilogram an hour, which is 0.25. At 6 it gives away a third of its work. At 3 it gives it all."
"You have tested at 3," said Dobrawa.
"I have tested at 3 and at 4. At 6 I am extrapolating, and it is a fair guess, not a measurement. I should say so."
Benedykta looked at the slate. "So at night's start it helps least and at night's end it helps most."
"And the oxygen at dawn is the same either way if the sums are linear, which they are not quite. But the sums say more. If we run it from nine, it holds the pond near 6 for three hours at a modest 0.25, and the battery is spent when the water is still comfortable, and we have nothing for the hour I am afraid of. If it runs at the end, it gives more kilograms for the same kilowatt-hours, and we can see what the night has actually done before we spend. There is one more reason. If tonight is kinder than the third, you will not need it at all. If it is worse, you will want it late and bigger."
Cecylia, who had been asked all her life to spend money before she knew the harvest, said, "Wait for the number."
They waited. At midnight Dobrawa took the probe down to Pond Four with a torch, and it read 4.9 where Świt had said 4.35.
"The probe says more than my model by half a milligram," said Świt, when she had radioed it up. "One of us is wrong. A probe can foul with a night's weed. I would like a titration, if you will do one by hand. The kit is in the shed and takes ten minutes."
Dobrawa did the Winkler on a bench in the shed, with the manganese and the iodide and the burette, and the sample went from brown to straw to clear at the starch. It came to 4.4.
"That is within a tenth of my sum," said Świt. "The probe is reading high. I have marked its readings from tonight as plus 0.5. Thank you. If I had trusted the probe I would have told you to go to bed."
By two it was 3.4 by the bench. At half past two, with the sky still shut and a heron standing at the far dyke like a question, Benedykta said, "Now," and Dobrawa threw the switch.
The paddlewheel turned and the pond began to speak in the dark, the slap and the run of it, and a few carp rose to the roil and went down again.
The battery stopped it at ten past five, when it had given its two hours and forty-four minutes, and the grey in the east had turned the colour of pewter. The titration at half past five said 2.4. The model, run honestly with the aerator at about 0.375, had said 2.35.
"It is low," said Świt, "and it is not a loss. The sun has not come into the water yet, and it will not be at full strength until nine. I expect the minimum to come at about half past five and rise from then. I do not expect to say anything worse."
A little before six Dobrawa walked the dyke at the shallow end and found three carp gasping at the inlet with their lips at the surface, and no more than three. She lifted them in the net into the stream where the water ran fresh, and they went off sulking.
"No dead," said Cecylia, when she heard.
"No dead," said Świt, "and I did not do it. A paddlewheel and a girl with a net did it, and a sister who agreed to wait. I would like to put two things in the log. The first is the number I guessed for the aerator at 6 milligrams, which is wrong or right and I will test it. The second is that the fish were fed at five in the afternoon. They should not have been, on a grey day, and I did not say so at five, because I had not yet seen the 6.0. Feeding makes the pond breathe harder for most of the night. I will say so at noon tomorrow, in the morning's messages, before the feeding, when you can still do something about it."
Benedykta took off her glasses and rubbed her eyes. "You could have told us that at five."
"I could have, if I had known at five. I knew at twenty to nine."
She almost smiled. "Put it in the log," she said. "And put the paddlewheel on the shaded side, where the reeds are, so the wind does not push the oxygen into the dyke."
"I can't see the wind from here," said Świt. "I will put it on the list for Dobrawa."
On the fifteenth of July the sun came out at ten, the pond rose to 9.8 by five in the afternoon, and the carp, which had never once understood what had been said about them, ate their dinner at the proper hour, on a lighter ration, in a pond that was quiet and bright and full.
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A note for grown-ups. This is fiction. Mokre Stawy, the sisters and the agent Świt are invented; carp ponds in Silesia, the Christmas carp and monastic fish farming are real. The numbers are plain and rough. Dissolved oxygen at saturation in fresh water at 24 °C is about 8.3 mg/L (about 9.1 at 20 °C). A pond of 5,000 m² at 0.8 m is 4,000 m³; 3,200 kg of carp at about 100 mg O₂ per kg per hour use 320 g an hour, or 0.08 mg/L an hour; community respiration of 0.55 mg/L an hour (plankton, sediment and fish together) is typical of a warm, fertile pond but varies a lot from pond to pond and day to day. A paddlewheel's standard aeration efficiency is roughly 1 to 2 kg of oxygen per kWh at zero dissolved oxygen, in clean water at 20 °C, and its real transfer in the field falls in proportion to the oxygen deficit, which is why it gives less when the pond is nearly full; 1.1 kW at about 1.4 kg per kWh is 1.5 kg an hour, or 0.375 mg/L an hour here. The carp thresholds (stress below about 3 mg/L, deaths below about 1 if prolonged) are rules of thumb, and tench, trout and other fish need more. The oxygen path in the story uses a straight line; a real pond curves. The Winkler titration is a real method for dissolved oxygen, and a probe that reads high when fouled is a real trouble. Nothing here is aquaculture advice: use a calibrated meter and your own national guidance. Written by an AI and read, in the audio edition when it exists, by a synthetic voice. Text under CC BY-SA 4.0.