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As far as I know, all land life depends on plants for survival. Plants convert sunlight to energy using photosynthesis, herbivores consume plants, and carnivores consume herbivores. All the energy is coming through the sun and being gradually processed by this food chain. If plants were removed from the system, all animals would eventually die as they would run out of energy.

Do fish and other water creatures also depend on plant life in the same way?

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Short answer

All marine life needs energy to survive and reproduce. "Heterotrophic" organisms get their energy from eating other organisms and digesting the molecules of their tissues for driving cellular respiration/function. Ultimately, though, entire ecosystems and food webs are dependent on the non-biotic energy (e.g., sunlight) captured by "autotrophs" (aka "primary producers") and stored in the molecules of their bodies.

Plants make up only a portion of the autotrophs present in the oceans. Photosynthesis by phytoplankton accounts for the majority of Net Primary Production (total energy stored by autotrophs minus energy used by autotrophs to live) in our oceans. Chemosynthesis of bacteria and archaea in the ocean depths account for a small degree of NPP, too.

Heterotrophic organisms eat the autotrophs, and higher trophic heterotrophs eat lower trophic-level organisms. Dead matter (still containing energy-rich molecules such as sugars and fats) drift to lower ocean depths to provide organisms farther from the photosynthetic surface regions with most of their energy. At all trophic levels, energy is used to drive cellular work or lost as heat; as such, less and less energy is available at higher trophic levels and therefore necessitates eating more and more food.

enter image description here

Illustration by Tim Gunther. Source: National Geographic

SUMMARY: all marine organisms are dependent on primary producers, most primary producers are photosynthetic, plants are responsible for a significant (though minority) amount of productivity in oceans, and phytoplankton account for the majority of energy capture in our oceans.

Long answer

The Basics

All biological systems on earth require energy and matter for survival. Primary producers are the organisms that acquire energy from non-biological sources to make them available to the rest of the biotic component of the ecosystem (i.e., their associated food web; sometimes visualized as an energy pyramid).

If all other organisms in an ecosystem depend on these primary producers as an energy source, it's important to try to quantify how much energy the autotrophs are converting from non-biotic sources and storing in their cells. We can quantify this in terms of productivity:

  • Gross primary productivity (GPP) is a measure of the conversion rate of the above non-biotic energy sources (typically solar) into stored chemical energy (typically in sugar molecules) by the autotrophs.

  • Net primary productivity (NPP) is essentially GPP minus energy used by the autotroph to survive (i.e., respiration).

enter image description here

Read here for more information about energy flow basics.

Primary Producers in Oceans

Although photosynthetic plants are the best known primary producers, they are not the only autotrophs on earth. In fact, the large portion of ocean productivity is a result of phytoplankton, a diverse collection of photosynthetic protists and cyanobacteria.

enter image description here

Image source: NASA, originally from Sally Bensusen, NASA EOS Project Science Office

The photosynthetic activity of plants and phytoplankton provide the majority of NPP for the entire ocean, but they are both limited in where they are located.

Sunlight is limited by water depth, reaching depths of less than 300 m.

https://disc.sci.gsfc.nasa.gov/education-and-outreach/additional/science-focus/ocean-color/images/spectral_light_absorption.gif

Image source: NASA

Since sunlight-availability dictates NPP to such a large extent, we can divide the ocean into vertical zones based on light availability. Only the top 200 m, referred to as the euphotic zone, has enough sunlight available for photosynthesis.

enter image description here

Image source: NOAA

Both of these photosynthetic organisms are also limited by nutrient availability.

  • For example, phytoplankton are most abundant in upwelling zones where sinking nutrients are brought back up to the ocean's surface. These occur both in the open ocean and along coastlines.

  • Plants, which absorb nutrients through root systems, are further limited by the availability of substrate within the euphotic zone. In other words, they need somewhere to "anchor" down. As such, plants are only found along continental shelfs or other shallow benthic or sedimentary regions within the euphotic zone. In other words, we would say plants are limited to coastal or littoral zones.

Although the majority of production in oceans is a result of euphotic photosynthesizers, very small amounts of production are the result of chemosynthesis at thermal vents and cold seeps in deeper ocean depths. the chemoautotrophs here are all bacteria or archaea.

NPP of our Oceans

So far we discussed the importance of net primary production (NPP), the autotrophs responsible for the ocean's NPP, and the limitations of where such autotrophs can live. The question still remains: to what degree do the oceans rely on these variable sources of NPP?

According to Fox et al. (2020), "Assessments of marine primary production...are made difficult by the extreme plasticity in phytoplankton physiology". However, certain broad trends are apparent.

Well, given that both plants and phytoplankton can exist in coastal/littoral zones, these zones (especially those associated with estuaries bringing in additional resources from freshwater systems), tend to have the greatest NPP per unit area of anywhere in the world.

However, since the pelagic, or open-water, portions of the euphotic zone make up such a large percentage of the earth's surface (~70%), this so-called epipelagic zone accounts for more NPP per year than any other biome or life zone globally just due to the immense area/volume for phytoplankton to live and photosynthesize.

  • Although phytoplankton make up <1% of the earth’s photosynthetic biomass, Field et al. (1998) estimated such phytoplankton are responsible for almost half the world's annual NPP.

George Kling (University of Michigan) provides 2 simple graphics demonstrating these two points on a course website of his here. I've copied lower quality images here for reference:

enter image description here

Modified from figures 4-5 of George Kling (University of Michigan) course page

Since only autotrophs can convert non-biologic forms of energy into chemical stored energy that heterotrophs such as fish and other water creatures can consume, all non-photosynthesizing organisms throughout the ocean's depths rely on the consumption of these autotrophs (or lower trophic-level heterotrophs) for energy. In fact, according to sigman and Hain (2012), usage of the energy captured but autotrophs and utilized by higher-trophic heterotrophs is so robust that all but <1% of the energy initially captured ever reaches the ocean floor as fallen debris/wastes/decomposing "snow".

As mentioned previously, a minor degree of chemosynthesis along the ocean floor exists, but such chemosynthesis adds minimal NPP to those zones. I could not quickly find a source that quantified the degree to which NPP of these deep zones comes from chemosynthesis (though see McNichol et al. (2018) for a fairly recent publication discussing and examining this topic). However, such chemosynthesis can only occur at thermal vents or cold seeps and therefore are quite limited in their spatial scope. Suffice it to say, that a very limited amount of NPP come from such chemosynthesis and the degree to which chemosynthesis adds to the NPP is spatially variable based on the presence/prevalence of inorganic energy sources such as thermal vents and cold seeps.


Further reading:

Chapter 17 "the Flux of Energy through ecosystems" in Begon, Townsend, and Harper. 2006. Ecology: From individuals to ecosystems. Blackwell Publishing, Malden, MA, USA.

Sigman, D. M. & Hain, M. P. (2012) The Biological Productivity of the Ocean. Nature Education Knowledge 3(10):21 [see here]


Citations:

Field, C. B., Behrenfeld, M. J., Randerson, J. T., and Falkowski, P. G. (1998). Primary production of the biosphere: integrating terrestrial and oceanic components. Science 281, 237–240. doi: 10.1126/science.281.5374.237

Fox, J., Behrenfeld, M.J., Haëntjens, N., Chase, A., Kramer, S.J., Boss, E., Karp-Boss, L., Fisher, N.L., Penta, W.B., Westberry, T.K. and Halsey, K.H., 2020. Phytoplankton Growth and Productivity in the Western North Atlantic: Observations of Regional Variability From the NAAMES Field Campaigns. Frontiers in Marine Science.

McNichol, J., Stryhanyuk, H., Sylva, S.P., Thomas, F., Musat, N., Seewald, J.S. and Sievert, S.M., 2018. Primary productivity below the seafloor at deep-sea hot springs. Proceedings of the National Academy of Sciences, 115(26), pp.6756-6761.

Sigman, D. M. & Hain, M. P. (2012) The Biological Productivity of the Ocean. Nature Education Knowledge 3(10):21 [see here]

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  • $\begingroup$ Great answer. Followup questions: I understand that unlike land plants, phytoplankton can move around. Does it do so strategically, like an animal? Does it run away from predators and such? $\endgroup$ – Ram Rachum Nov 30 '20 at 10:37
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    $\begingroup$ @Ram most phytoplankton are non-motile, and simply move along with the ocean currents. I suppose dinoflagellates can move minor distances with the use of their flagella, but they are so small that I'm not aware of any intentional mass movement of phytoplankton worth noting. See here and here for more info. $\endgroup$ – theforestecologist Dec 1 '20 at 20:03
  • $\begingroup$ @Ram although phytoplankton don't generally move strategically, the heterotrophic zooplankton that feed on them do: nature.com/scitable/blog/saltwater-science/… $\endgroup$ – theforestecologist Dec 1 '20 at 20:08
  • $\begingroup$ They're heterotrophic though, so they still maintain the rule of "a creature that does photosynthesis can't move" $\endgroup$ – Ram Rachum Dec 1 '20 at 20:19
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Yes, photosynthesis provides nearly all of the ocean's biomass and energy. The best is to study oceanic food chains and webs.

enter image description here

It looks like there aren't enough plants in the ocean because they don't form huge reserves of foliage and branches... 10% of land plant mass dies and is replaced every year, and 100% of oceanic plants die and are replaced every week:

"Biologists set out to estimate the total biomass of phytoplankton and calculated that less than one billion tonnes of the single-celled microorganisms were alive in the ocean at any one time. There were 45 billion tonnes of new phytoplankton each year, 45 times more than their own mass at any given time. The phytoplankton would therefore have had to reproduce themselves entirely, on average, 45 times a year, or roughly once a week. In contrast, the world's land plants have a total biomass of 500 billion tonnes, much of it wood. The same calculations showed that the world's land plants reproduce themselves entirely once every ten years. https://www.nature.com/articles/483S17a

Every step of the marine energy pyramid has about 10% efficiency, so the shrimps and zooplankton take about 10% of the phytoplankton energy, then small fish 1%, and big fish 0.1% of the plant energy. For every 500,000 units of radiant energy that hit the ocean surface, 2% is converted to phytoplankton at the base of the trophic pyramid, and the 500,000 units are necessary for every 10 units that a tuna consumes. https://oceantracks.org/sites/oceansofdata.org/files/energytransfer.png

https://en.wikipedia.org/wiki/Marine_food_web

In biology, we typically study coccolithophores and phytoplankton to learn the marine food web.

Areal map of sea chlorophyll in Mg/m2.

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    $\begingroup$ @theforestecologist I've found a quantitative estimate of the biomass of phytoplankton in the oceans, it's 1 billion tonnes of phytoplankton at any one time versus 500 billion for terrestrial plants including their wood. nature.com/articles/483S17a $\endgroup$ – aliential Nov 29 '20 at 18:34
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    $\begingroup$ @theforestecologist The researchers said that the average kg of oceanic vegetation survives 7-8 days, and the average kg of terrestrial vegetation survives about 10 years. The photosynthetic activity of all the lands and all the seas are about the same, a bit higher for terrestrial vegetation. $\endgroup$ – aliential Nov 29 '20 at 18:44

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