Reconstructing a marine ecosystem
[40 minutes]
Here’s the scenario I had in mind when I created the data – but what is important is that the students think for themselves and come up with a meaningful scenario that can account for the properties of the data.
A: Essentially a river-mouth deposit. Floods move cobbles – it’s an erosive setting. No “sediment” as such so difficult to be truly infaunal. High sedimentary setting will crush animals – only tough bits will survive when they fall between cobbles, and even then few will be identifiable. Epifaunal creatures might have tougher shells so be more likely to be preserved in sediment.
B: Outflow of the sewage pipe. There’s currently sewage gushing out, creating an algal bloom that sucks all the oxygen out of the water. That creates stress which reduces diversity and kills anything infaunal. Suspension feeders will be feeding of falling algal particles, which will be raining out faster than they can be eaten. The death assemblage captures life when there’s not sewage outflow too. At such times, there’s still a lot of dead algae on the sea floor; these provide plenty of nutrients (hence high diversity), mainly available to deposit feeders (there’s some energy from tides perhaps, but not too much – perhaps this is the Baltic basin, where tides are absent and storms few. Organic rich sediment consumes oxygen, making deep burrowing difficult – life is mainly epifaunal.
C: We’re some way off the mouth of the river but still exposed to freshwater; this lower salinity keeps diversity low. Diversity’s higher in the sediment because life from elsewhere gets washed in, and when the river’s not flowing very hard or its course is deflected by long-shore drift, ‘normal’ marine life can move back in. River discharge keeps energy levels high, favouring suspension feeders. Plenty of oxygen and clean soft sediment makes infaunal life a must.
D: This is the “background sediment” representing ordinary shoreface conditions. There are plenty of species – more in the death assemblage due to time averaging. Storms and waves keep much food in suspension (for deposit feeders) but quiet days are frequent enough for deposit feeding to be profitable too. The water is well oxygenated and the sediment soft, so infaunal is abundant.
E: Downstream (longshore drift) of sewage discharge there is an excellent supply of food (dead algal material) without being subjected to the eutrophicated conditions at the mouth of the pipe. Dark mud is organic rich; food comes faster than it can be suspended, leaving nutrient-packed sediments ripe for deposit feeding (though the current bloom makes suspension feeders well represented at the present moment). There’s a down-tick in oxygen at the moment leading to a slight proclivity to epifaunal feeders, but infaunal is the norm as sediments are soft and oxygen is available.