Estuaries and the Salt Wedge

Last week we made a "miniature ocean ecosystem" containing just salt water. Let's start by hearing student reports on the present conditions, and observations over the past week (e.g. condensation and "rain", poop, ...)

The shrimp can also live in (almost) fresh water ("brackish" - just a little salt present). In estuaries like the New York harbor area (including the East River), salt water meets fresh water. Let's make a miniature estuary and study it; then see what happens when shrimp move in!

Our study will be scientific - we'll make and record careful observations, try to explain what we see, use our explanations to predict what else might happen, then test our predictions. It takes patience to get really good results, but it's worth it!

Materials

• table salt (NaCl)
• water
• food coloring (or phytoplankton)
• 2 (or more) clear plastic glasses; empty 20 oz. soda bottles with caps, marked to ~50 ml.
• tablespoon measure, graduated cylinder, floating hydrometer
• more brine shrimp and phytoplankton
• potato pieces (for flotation tests)
• PENCIL and PAPER!! (recording is important!!)
• paper towels in case of a spill

Procedure

• fill one glass (the one with tape on it) about 2/3 way with tap water
• pour from the glass into the bottle, to the BLUE line
• pour out from the bottle into the untaped glass
• pour from the taped glass into the bottle to the blue line
• dump any extra from the taped glass
• put 1 tbsp NaCl in the soda bottle, shake til dissolved. Describe what happens
• put the salt water into the taped glass; add a small piece of potato to each glass, observe and record
• remove the potato pieces
• quick demo of hydrometer in fresh water (vs. salt water later)
• add some food coloring to make the very salty water dark (other groups: color the fresh water)
• very gently pour the salty water into the fresh water (other groups: pour fresh water onto the salt water)
  (it helps to tilt the fresh water glass to slow the pouring, and do it VERY SLOWLY)
• observe and record the results!
• Why does the salty water go to the bottom?
• collect all group's salt water into cylinder, show hydrometer again, note difference
• make demo salt wedges (one w colored fresh, one w colored salt) using phytoplankton as the coloring agent
• add a few shrimp to each (w/o disturbing the stratification), observe what happens over the next week
• also set up a shrimp nursery with brine (clear) and eggs; observe over the next weeks - watch for babies!

When ocean water comes into an estuary with the tide, it tends to settle to the bottom just like our model. As it pushes its way in, it usually gets a wedge shape - hence the title of this Web Site!