Dear River

Dear Mississippi River,
This is where you're allowed to be.

http://media.nola.com/hurricane_impact/photo/hurricane-protection-system-graphic-8def260a8445ece9.jpg

Any questions?  Please refer to the Army Corps of Engineers.

Sincerely,
People

Ocean science questions ANSWERED! post 9 - Why is the sea salty?

Q:  Why is the ocean salty? And what are the benefits of its saltiness? (Emma, Paul)

A:  Yayyyy!  I love this question – it seems so simple, and then there are all of these nuances that become important and it’s not a simple story.  So here goes:  The ocean is salty because it slowly accumulated salts over the course of its formation, and with the formation of the hydrologic cycle of the earth.  Nowadays, there is a tidy balance between th continuous onslaught of salts being delivered from rivers, and the output (export or sequestration) of those materials.  On land, rocks are weathered through chemical and mechanical weathering that dislodges minerals from the rocks and mobilizes them into runoff waters such as rain, rivulets, groundwater and rivers and streams.  This weathered material can take the form of dissolved salts or suspended particles (sediment).   Regardless, a huge amount of sediment and dissolved minerals are delivered to the ocean at the mouths of rivers.  There are other sources of salt material, as well:  dust mobilized from the great deserts and deposited in the ocean, hydrothermal vents and underwater volcanic eruptions, for example.  It seemed counter-intuitive to me that this bit of salt could make a difference in the enormous ocean,  but we’ve used this idea of salts input and outputs we see today to explain how salty must have formed on early earth.  Some of the ocean’s salts – especially chlorine – are extremely soluble, and extremely unlikely to be removed from the water column once they’ve entered.  The water in the ocean does not mix and turn over very quickly, so the salt eventually accumulated, but it took time.  The commonly held belief now is that salts built up in the ocean over a very long time period, enough to make the water salty, and now the ocean is in a relative steady state with respect to salinity (the latter is not disputed, and there is a huge amount of evidence that shows that salinity has been stable for millions, maybe billions of years).  When ocean water evaporates, the salts are left behind.  Back to present day ocean:  there is a net balance of salt in and salt out, while water continues to flow in and evaporate out, and we experience the salty ancient ocean.   Some of the salts are taken out of the water by organisms (especially calcium carbonate formation), by volcanic activity on the seafloor, and by the subduction of tectonic plates, but these are all slow processes. 

The salinity of the ocean ranges from brackish in the estuaries and some bays, with a salinity of less than 1, to high salinities in a place like the Mediterranean Sea with salinities upwards of 38 (parts per thousand).  Most of the ocean ranges from between 32 – 36.  The units used to describe salinity are interesting, because we don’t use a measure of actual salt per volume water anymore.  We now use the conductivity of the water, which indicates the level of salts, and the measure is a unitless ratio, so sometimes we use “PSU” to denote practical salinity units, but it’s not necessary to use units and sometimes it’s frowned upon. 

The ocean being salty is important, because we wouldn’t have life as we know it without it!  This is because of the hydrologic cycle.  If we didn’t have our hydrologic cycle, we wouldn’t be here.  We rely on the cycle for our water, and everything we eat depends on it too.  The ocean and the land are inextricably connected by the water and biogeochemical cycles of the planet, and we need them all to function. 

Ocean science questions ANSWERED!: post 8 - Deepwater Horizon and oil spills

      Q:  What's the truth about oil spills & the dispersant chemicals used to "clean" them up? (several people asked about this)

A:  The truth is similar to Fukushima.  The Deepwater Horizon blowout is one of the worst environmental disasters to take place in the USA – and the biggest oil spill in US waters.  There is a lot of hype though too.  Its horrible and jarring to see images of the extent of the oil – and it was massive – from DWH.  One of the things is:  oil floats.  So most of the material from the spill was cruising for the surface, and still being less dense than water it gets blown to shore very easily.  SO.  The dispersant was a good idea for saving the large, charismatic, and commercially important species, because it binds to the oil particles and makes them sink.  There are far fewer organisms living on the bottom of the deep sea than the coastal zone.  If there were dispersants used in the coastal zones (and shallow areas), that would probably be disastrous because there is so much life there.  But in the open water and deeper sea, closer to the wellhead, it’s a decent option because it gets that oil out of the most productive part of the ocean – the surface layers.  No matter what, the oil is impacting some organisms – it’s a matter of how much and where.  There has been a lot of talk about the dispersants, and I still don’t know the answer – I haven’t been shown anything to prove to me that those chemicals aren’t horribly toxic.  But the issue seems to be that we are continuing to approve new drilling permits for deep sea operations, and BP and the companies responsible are not doing their share to take responsibility for the impact the spill had on the ecosystem.  I’m really interested in studying the impacts to the benthos of the oil spill and dispersants – my guess (and what I’ve read so far) is that the benthic community was drastically altered after the oil spill, although we don’t have conclusive proof yet that its directly because of the oil and dispersants.  So the truth is, more needs to be done to find out the real environmental costs of the oil spill.  

Ocean science questions ANSWERED! post 7 - Fukushima radiation

      Q:  What's the truth about Fukushima radiation landing on US shores?

A:  The truth is that radiation has not, as far as we know, gotten to the west coast of the US, but there are imminent plans to test the waters.  There is still radiation in the water from every atomic bomb that’s ever been tested – the stuff stays around a long time – but that doesn’t mean there’s radiation poisoning everything.  So it’s a myth that there is a health problem right now for US-dwellers, but this is always a moving target and may be revised as new information becomes available.  What is NOT a myth is that the Fukushima disaster was unparalleled in its disastrous effects.  The good folks at Southern Fried Science have done a nice corralling of info for us (also see the Deep Sea News pieces – very good):  http://www.southernfriedscience.com/?p=16363.  

Ocean science questions ANSWERED! post 6 - Salps and pteropods! Gelatinous zooplankton!

Q:  What is the overall niche of salps and pteropods? Are there any theories to predict where you get a food web with lots of gelatinous little zooplankton (I'm not counting medusae) instead of crustaceans?  (Neil)

A:  Salps have a particular niche, because there are only a few species groups and they tend to feed in a similar fashion to one another – they make large aggregates and feed and respire and excrete and die in a big cluster.  Pteropods on the other hand are varied, and diverse – from thecosome pteropods (the little “sea snails” with shells made so famous by ocean acidification research) to gymnothecates with no shell and soft bodies, they are varied in form, feeding and life cycle.  I guess there might be a similar niche, in the sense that shelled-pteropods make a mucous net to stick prey onto and then consume the house, and salps do something similar.  They both filter a large amount of water, although salps probably more because they are bigger in size. 

Theories to predict where you’d get a food web with lots of LITTLE gelatinous zooplankton!  Yes.  Making a theory up on the spot, I think there could be more LITTLE gelatinous zooplankton in the highly productive upwelling region food chains.  I’m saying that because we usually see those areas with a simplified food chain, with forage fish feeding on zooplankton, and large animals feeding on the fish – sort of skipping over the large jellies and some of the larger macrozooplankton.  Of course, systems are always more complicated than the model, but that could be a first crack at my theory.  Also, shelled pteropods have been shown to be a significant part of salmon’s diets (but only in some years).  So that’s not much to build a theory on, either.  Still a long way to go, in my view, for a unified theory on zooplankton ecology – especially gelatinous ones.  

Update:  i just read that Oikopleura (larvaceans) have mucous nets with such tiny openings that they can snare bacteria in there.  !  This seems mind-boggling to me, but what it means is that maybe Oikopleura are playing a bigger role in the microbial loop than we know at the moment.  If that's the case, perhaps there's a truncated food web going on when Oikopleura are present in large enough numbers.
Update #2:  it seems that some salps swarm and aggregate when other's (meso+macrozoop) don't, usually because of nutrient limitation.  This might also be a suggestion of a niche for salps:  if they can filter and feed on bacteria and the microbial loop, they might be able to subsist without phytoplankton if there is enough dissolved organic matter/dissolved organic carbon that's floating around in the water.  Such a scenario could happen if there were a phyto bloom, and then nutrients were limited and there wasn't a complete drawdown of nutrients by zooplankton, then phytos die off and there's just a slow accumulation of DOC for a period of months.  However, this doesn't really suggest a "niche" to me, per se, because it seems like salps could/would just as easily swarm in an environment replete with nutrients and things to eat.  There is probably a competitive advantage to being able to swoop in and eat when others can't, but i'm not sure that constitutes a niche.
Update #3:  i think this exploitation of resources by salps could be tied to disturbance - if there's disturbance and enough DOC is in the water, salps can ride in.  

Ocean science questions ANSWERED!: post 5 - Gulf of Mexico

      Q:  How has past and current geology, historic ocean currents and other factors influenced the diversity of the GOM? (Matthew)

       A:     First, lets discuss the geology of the Gulf of Mexico.  The Gulf is considered a passive margin, because its not at the site of an active fault or tectonically active rift, subduction zone or volcanic feature.  It is also dominated by the enormous Mississippi River flowing into it.  The continental shelf varies in length around the Gulf – it extends the farthest around Florida (the Florida Keys are all on the shelf).  The basin itself is also relatively shallow, compared to deep ocean basins.  These features all affect the type of sediment and substrate found at the bottom of the Gulf of Mexico – mostly reefs (around Florida) and soft-bottom substrate.  Also, along the northern Gulf continental slope (that’s the steep part between the shelf and the basin, usually about 200 – 3000 m water depth), there are extensive salt domes and carbon-rich deposits embedded in the slope, shelf and continental rise.  This is where all the oil and gas drilling and mining occurs.  Oil and gas deposits suggest that parts of the Gulf of Mexico were filled with carbon-rich deposits (such as plants and animals in abundance living, dying, and becoming deposited on the substrate and subsequently buried).  Such a situation could have occurred with low stands of the sea, where sea level was much lower than it is now (up to 200 m lower), and rivers emptied directly onto the continental shelf.  In such a low stand of the sea, organic deposits could build up and eventually become fossilized or sedimented.  When sea level rose again, the shelf would be covered with water and marine sedimentation would begin again, and thus we see layers of carbon rich deposits and marine sediments. 

      The canyons of the slope and rise are built by turbidity currents.  Turbidity currents are gravity flows of sediment, mobilized with water and remain in motion because of the turbidity.  Turbidity currents move because the sediment is much more dense than water, and so the turbidity of a dense material creates a flow that accumulates as it “runs”, and turbidity currents can blast through the topography at extraordinary speeds.  Turbidity currents leave “turbidites” behind in their wake, or sediments from turbidity currents.  These turbidites resemble riverine delta regions in terms of deposition grain size – the large grains settle out first, then silts and clays last.  Turbidity currents and turbidites are important features of the GoM. 

      As for biodiversity in the Gulf of Mexico, there are connections with the geology and physical oceanography of the basin, as these are often the driving factors behind diversity.  However, also playing a role in biological production are the availability of nutrients and appropriate habitats for each trophic level.  The benthic organisms are the most diverse within the marine species, so if there are good habitats and growth conditions for benthics then there will be high diversity. For this reason, some have chosen to measure substrate diversity as a proxy for biodiversity of the system.  In reality, there are more factors than this contributing, especially including nutrients and recruitment ability of each population.  However, the Gulf plays host to some of the most diverse benthic ecosystems on the planet:  coral reefs.  Reef systems tend to have the most diversity of any marine ecosystem type, the “tropical rainforest” of the sea.  Since the Gulf of Mexico does have some coral reefs, especially in the shelf area off Florida, it can be considered to have relatively high diversity.  Also, as the Gulf of Mexico is NOT a big deep basin with unproductive oceanic gyres in it, the whole of the basin is relatively productive.  This doesn’t necessarily mean its more diverse, but more productivity is a good starting point for being able to have high diversity.  There are smaller mesoscale eddies that transport plankton, nutrients and fish larvae around the basin, along with currents, but there are no upwelling or downwelling features that impact biological productivity on a large scale.  The “global conveyor belt” of thermohaline circulation is such that the Gulf receives surface (warmer, less nutrients) water from the Atlantic, but probably none of the North Atlantic Deep Water or nutrient and oxygen-rich deep waters.  Biological production is limited to the nutrients that are provided via the rivers, Aeolian input and from marine sediments.

Ocean science questions ANSWERED!: post 4 - Submarine groundwater discharge

      

      Q: I have often wondered: if there is "groundwater" beneath the earth's surface, there must also be groundwater beneath the ocean floor too. Is it true? How deep does it go? Is it all salt water? How salty? (Marshall)

A:  Yes, its true!  Well, to a certain point.  When you say, “ocean floor”, think of the shoreline.  That’s where the groundwater is.  There’s a very new, very exciting line of study of groundwater discharge.  Its fascinating, and its possible that there is a significant amount of water exchange between the fresh and saltwater bodies that we really didn’t know about before.  It doesn’t go all that deep – maybe 100 m or so.  And it is certainly not all saltwater – using radon as a tracer, people have shown that there are significant amounts of freshwater seeping out.  However, saltwater can also seep into the freshwater, and there are freshwater aquifers that are often an important source of drinking water for coastal communities, that are becoming contaminated by saltwater intrusions.  This is exacerbated by draining the aquifers:  if you’re pulling water out, laws of physics indicate that new water will fill in, and saltwater may be more readily available, especially at the ocean’s shoreline.  So the water may be saline, fresh, or brackish (usually salt water is defined as having above 20 parts per thousand of salt, whereas freshwater has zero).  Also, in the deep ocean at the hydrothermal vents and spreading centers (the Mid-Ocean Ridges) there is significant water exchange into the rocks and sediment.  This is all saltwater, though.  

Ocean science questions ANSWERED!: post 3 - Tides

          I wanted a little exercise to help me study for my comprehensive exams in my PhD program in Marine Science.  So i asked you all to write to me with your burning ocean science question, and i will answer right away, in practice for the exam.  Here are some of the q's and a's.

     Q:  When low tides happen, is the "middle" of the ocean higher? How can it not 'fall' back toward the shores?

A:  I love this question – I had the same question in my mind when trying to grapple with the idea of tides!  Tides are the product of gravity, and the force of gravity exerted by the moon, and the sun.  And for each of these heavenly bodies, there are two forces to consider:  gravitational force (gravity) and centrifugal force.  For gravitational force, the water on earth is pulled toward the sun, toward the moon.  For centrifugal force, the water on earth is pulled away from the sun, away from the moon.  So now envision the Earth, orbiting around the sun, and picture a lens around the sphere of the earth.  Imagine the earth at 3 o’clock, with the sun in the center, and at 9 o’clock – the gravitational force pulls the water to opposite sides of the earth at each of those times.  Now the gravitational force is stronger with the sun (the moon is closer to earth than the sun, but the sun is MUCH bigger), but both forces play a role in each other’s gravitational forces and the tides.  SO. You have the earth orbiting the sun.  And you have the moon orbiting the earth.  And you have the earth spinning on its axis each day, as well as the annual orbit of the sun.   So there’s a LOT of motion going on, and this is why the tides are so complicated at each given place! 

To answer your question – yes!  When its low tide on Muir Beach, the “middle” of the ocean is higher – out in the deep Pacific.  However, thanks to our gravity field on earth, and the fact that the ocean basins are so enormous, the basins essentially absorb that bulge.  But this also creates high tide somewhere else – with the Muir Beach example, lets say it’ll be high tide at Fukushima, maybe, at the same time.   Essentially, the water moves like this because of the amazing gravitational fields of the earth, sun and moon, and because of centrifugal forces.  

Ocean science questions ANSWERED! post 2 - Ocean Acidification

      I wanted a little exercise to help me study for my comprehensive exams in my PhD program in Marine Science.  So i asked you all to write to me with your burning ocean science question, and i will answer right away, in practice for the exam.  Here are some of the q's and a's.

    Q:   What's the deal with ocean acidification?  

      (original question: How has human-caused greenhouse gas build-up affected the acidification of the seas--or is it vice-versa? or separate events convergent? - Qayyum)

A:  Ocean acidification is the process whereby the ocean becomes more acidic with increasing carbon dioxide (CO2) in the atmosphere.  Ocean acidification is an impact to the carbon system, or carbon cycle.  CO2 is the most prevalent greenhouse gas; that is, it’s the one found in highest concentrations in the atmosphere as compared to the others.  The ocean is the largest sink (sequestration site) of CO2, for any entity in continual contact with the atmosphere; the ocean captures and stores about 1/3 of the human-produced carbon dioxide today. This is due to phytoplankton taking CO2 in and eventually sinking (in one form or another) to the bottom and becoming sediment.  There is this magnificent cycle of carbon going on – CO2 exchanges with seawater due to diffusion and photosynthesis via phytoplankton (algae).  In order to understand the details, you need to understand some chemistry.  First lets discuss pH:  pH is a measure of hydrogen ions.  Hydrogen ions are sort of unusual, because they are positively charged and “want” to bond with other things.  The activity of the hydrogen ions in the water is pH, or acidity.  More hydrogen, more acidity.  Now lets get back to CO2.  Once in the seawater, CO2 goes through several different reactions interacting with water (H2O), and in each reaction hydrogen ions are produced.  So on a very basic level, more CO2 coming into the water will mean more acidity.  Its complicated by the remainder of the carbon cycle and the sequestration of carbon mentioned earlier.  In order for pH to stay the same, with increased CO2 in the water, there would need to be increased sequestration (input = output).  Sequestration is accomplished by plants and animals incorporating the CO2 into their bodies in the form of calcium carbonate.  The plants/animals die, and the calcium carbonate sinks and becomes sediment, sequestering that carbon in the sediment.  SO, one problem is that some of the organisms that create calcium carbonate structures in their bodies have a harder time doing that when there’s more CO2 in the water.  Furthermore, there is less deposition of calcium carbonate to the bottom, if there is more CO2 in the water.  This is all to say that there are several feedback loops that make more CO2 in the water feedback on itself and fuel more and more acidity.  There is much more we need to know about this process, and how increased acidity might affect organisms, AND the process of changing pH over the long term is VERY slow.  It makes the fact that we are seeing a change in pH at all in the ocean very troubling, and urgent in terms of understanding more about what’s going on.  

Ocean science questions ANSWERED! post 1 - Temperature rising

I wanted a little exercise to help me study for my comprehensive exams in my PhD program in Marine Science.  So i asked you all to write to me with your burning ocean science question, and i will answer right away, in practice for the exam.  Here are some of the q's and a's.

Q:  How does the rise in global temperatures affect the carbon-carrying capacity of the oceans? Can you quantify this? (Bruce)

               

A: Yes!  This is an important component of ocean acidification.  OA is the process whereby the ocean becomes more acidic with increasing carbon dioxide (CO2) in the atmosphere.  Ocean acidification is an impact to the carbon system, or carbon cycle.  There is no “carrying capacity” of carbon in the ocean per se; that is, it is unknown how much carbon the ocean could store or sequester.  The ocean is the largest sink (sequestration site) of CO2, for any entity in continual contact with the atmosphere; the ocean captures and stores about 1/3 of the human-produced carbon dioxide today. This is due to phytoplankton taking CO2 in and eventually sinking (in one form or another) to the bottom and becoming sediment.  The way temperature works into all of this is twofold.  First, CO2 is a greenhouse gas.  This means that the more CO2 there is the atmosphere, the more solar radiation will be trapped in the atmosphere, and the warmer the temperature will get at the surface of the Earth.  So it’s a tidy little feedback loop.  Second, temperature affects the solubility of gases (the amount of the gas that can be present in the water).  Higher temperature means less gas solubility.  This isn’t so much a “problem” for absorbing CO2, and transporting to the sediment, since higher temps will mean less CO2 in the water, but it is a problem for absorbing oxygen!  Higher temperatures mean that over the long term, less oxygen-rich waters will be formed, and oxygen rich waters are one of the VERY important components of fueling phytoplankton growth in the most productive parts of the ocean. 

               So to answer your question, yes, rising temperature definitely impacts the ability of the ocean to sequester carbon dioxide.  And yes, we can measure it!  Doney et al (2009) provide an excellent review of the carbon cycle in the ocean, and how rising CO2 levels can be measured.  Measuring global temperature is continually occurring, and is also accomplished by measuring levels of greenhouse gases in the atmosphere.