Aquatic Ecology Course Zoo 374

1- Dissolved Gases The gases dissolved in sea water are in constant equilibrium with the atmosphere but their relative concentrations depend on each gas' solubility, which depends also on salinity and temperature. As salinity increases, the amount of gas dissolved decreases because more water molecules are immobilised by the salt ion. As water temperature increases, the increased mobility of gas molecules makes them escape from the water, thereby reducing the amount of gas dissolved.

1- Dissolved Gases (cont.) Nitrogen , oxygen and CO2 are the most aboundant & important gases dissolved in s.w Nitrogen: Represents 64% of total dissolved gases in s.w. Almost inert i.e. has very low solubility and chemical reactivity. Nitrogen requirements of marine organisms must be met by nitrogenous compounds (dissolved – particulate – dissolved plant & animal tissue in the sea).

Oxygen: * Its deficiency is fatal to aquatic organisms, i.e. many fish died not from the direct toxicity of pollutants but due to O2 consumption in pollutants biodegradation. Presence of O2 is fatal to other organisms (anaerobic bacteria). Oxygen sources: 1- mainly from the atmosphere (The solubility of O2 is directly proportion with the atmospheric pressure & inversely proportion with water temperature and salinity) 2- secondly, the photosynthesis.

CO2: Used by plants during photosynthesis. Affects the acid-base balance in s.w by forming bicarbonate and carbonate ions under varying conditions [ Buffering action ]. These interactions can become quite complex, and have major impacts on the distribution of life in the ocean.

1- Dissolved Gases (cont.) All gases are less soluble as temperature increases, particularly nitrogen, oxygen and CO2 which become about 40-50% less soluble with an increase of 25ºC. When water is warmed, it becomes more saturated, eventually resulting in bubbles leaving the liquid. Fish like sunbathing or resting near the warm surface or in warm water outfalls because oxygen levels there are higher. The elevated temperature also enhances their metabolism, resulting in faster growth, and perhaps a sense of wellbeing

Likewise if the whole ocean were to warm up, the equilibrium with the atmosphere would change towards more CO2 (and oxygen) being released to the atmosphere, thereby exacerbating global warming.

2- Major constituents: ( elements have concentration> 1 mg/l in s 2- Major constituents: ( elements have concentration> 1 mg/l in s.w): Only six elements and compounds comprise about 99% of sea salt: Chlorine Sodium Sulfate Magnesium Calcium Potassium

Salinity and the main salt ions The salinity of sea water (usually 3.5%) is made up by all the dissolved salts.

N.B. The salinity of the oceans changes slightly from around 32ppt to 40ppt. Low salinity is found in cold seas, particularly during the summer season when ice melts. High salinity is found in the ocean 'deserts' in a band coinciding with the continental deserts. Due to cool dry air descending and warming up, these desert zones have very little rainfall, and high evaporation. The Red Sea located in the desert region but almost completely closed, shows the highest salinity of all (40ppt) but the Mediterranean Sea follows as a close second (38ppt). Lowest salinity is found in the upper reaches of the Baltic Sea (0.5%). The Dead Sea is 24% saline, containing mainly magnesium chloride MgCl2. Shallow coastal areas are 2.6-3.0% saline and estuaries 0-3%.

Salinity and the main salt ions Salinity affects marine organisms because the process of osmosis transports water towards a higher concentration through cell walls. A fish with a cellular salinity of 1.8% will swell in fresh water and dehydrate in salt water. So, saltwater fish drink water copiously while excreting excess salts through their gills. Freshwater fish do the opposite by not drinking but excreting copious amounts of urine while losing little of their body salts. Marine plants (seaweeds) and many lower organisms have no mechanism to control osmosis, which makes them very sensitive to the salinity of the water in which they live.

Density The density of fresh water is 1.00 (gram/ml or kg/litre) but added salts can increase this. The saltier the water, the higher its density. When water warms, it expands and becomes less dense. The colder the water, the denser it becomes. So it is possible that warm salty water remains on top of cold, less salty water. The density of 35ppt saline seawater at 15ºC is about 1.0255, or s (sigma)= 25.5. Another word for density is specific gravity.

3- Minor elements: ( elements and compounds have concentration < 1mg/l) - Nutrient salts: Nitrogen compounds include: 1- Dissolved inorganic nitrogen(DIN) represented by: Nitrate(NO3), Nitrite(NO2), Ammonia(NH3). 2- Dissolved organic nitrogen (DON). 3- Particulate nitrogen (PN).

- Nutrient salts: Phosphorous compounds include: 1- Dissolved inorganic phosphorus (DIP) Mainly orthophosphates (H3 PO4 ) 2- Dissolved organic phosphorus (DOP). 3- Particulate phosphorus ( PP ). Importance of phosphorus as a nutrient element: 1- It is the limiting element for the production of aquatic environment. 2- It is a part of DNA & RNA molecules [ molecules that store energy (ATP & ADP) ] 3-Ca phosphate is a building block for bones & teeth. 4- Phospholipids found in all biological molecules.

N.B. The main nutrients for plant growth are nitrogen (N as in nitrate NO3-, nitrite NO2-, ammonia NH4+), phosporus (P as phosphate PO43-) and potassium (K) followed by Sulfur (S), Magnesium (Mg) and Calcium (Ca). Iron (Fe) is an essential component of enzymes and is copiously available in soil, but not in sea water (0.0034ppm). This makes iron an essential nutrient for plankton growth. Plankton organisms (like diatoms) that make shells of silicon compounds furthermore need dissolved silicon salts (SiO2) which at 3ppm can be rather limiting.

Deep sea temperature, oxygen & nutrients In this diagram one can see how light penetrates no deeper than 150m for photosynthesis. Indeed at 800m, the ocean is pitch dark. In the surface mixed layer above the thermocline, water mixes sufficiently to sustain life. Gas exchange with the atmosphere is near-perfect such that the oxygen concentration in the water is in equilibrium with the atmosphere. But it rapidly decreases below 50-75m as photosynthesis declines while animals use up most oxygen. At around 800m oxygen levels reach a minimum (as also carbondioxide levels reach a maximum, not shown). Towards the deep and bottom water, oxygen levels increase slightly due to an influx of cold bottom water from the poles. Due to lack of oxygen, deep sea fish cannot be very active.