Rinse split peas. In a large pot bring 8 cups of water to boil with split peas and spareribs. Skim off any floats. Pour off and rinse peas and meat with water, then put peas and meat back in pot, add 8 cups of water and bring to boil again. Add carrots, leeks, onions, celery, and vegetable broth cubes to pot and let slowly simmer with the lid off for two hours until the soup thickens and peas are done. Lift meat from the pot, remove bones, cut meat into small pieces, and return to pot. Add whole sausage and warm through, 15 minutes. Season with plenty of salt and black pepper. To serve, cut sausage into one piece per portion, sprinkle chopped celery leaves over soup, and offer with slices of pumpernickel.