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Freshwater Ecosystem - an overview

source : sciencedirect.com

Freshwater Ecosystem – an overview

II. Diversity of Habitats

Freshwater ecosystems consist of entire drainage basins as water moves from land and in groundwater runoff to stream and river channels, and to recipient lakes or reservoirs. The nutrient and organic matter content of drainage water from the catchment area is modified in each of the terrestrial soil, stream, and wetland–littoral components as water moves downgradient to and within the lake or reservoir itself (Fig. 2). Photosynthetic productivity of organic matter is generally low to intermediate in the terrestrial components, highest in the wetland–littoral interface regions between the land and water, and lowest in the open water (pelagic) zone. The same productivity profile emerges in the gradient from land to river channels, where the greatest productivity occurs in the marginal floodplain regions. Autotrophic productivity in river channels is generally low, as is also the case in the pelagic regions of lakes. Most of the organic matter utilized by heterotrophic communities in running water is imported from floodplain and terrestrial sources as particulate and especially dissolved and colloidal organic compounds.

Figure 2. The lake ecosystem showing the drainage basin with terrestrial photosynthesis (PS) of organic matter, movement of nutrients and dissolved (DOM) and particulate (POM) organic matter in surface water and groundwater flows toward the lake basin, and chemical and biotic alteration of these materials en route, particularly as they pass through the highly productive and metabolically active wetland–littoral zone of the lake per se (net organic productivity in metric tons per hectare per year). (Modified from Wetzel, 1990).

The interface region between the land and water is always the most productive per unit area along the gradient from land to the open water of lakes, reservoirs, and streams. Because most aquatic ecosystems occur in geomorphologically mature terrain of gentle slopes and are small and shallow, the wetland–littoral components usually dominate in both productivity and the synthesis and loading of organic matter to the systems. The region of greatest productivity is the emergent macrophyte zone. Emergent aquatic plants have a number of structural and physiological adaptations that not only tolerate the hostile reducing anaerobic conditions of saturated sediments but also exploit the high nutrient conditions and water availability of this habitat. Nutrients entering the zone of emergent macrophytes from external sources tend to be assimilated by bacterial and algal microflora of the sediments and detrital organic particles, and are then recycled to the emergent macrophytes. Dissolved organic compounds released from decomposition of plant detrital materials dominate the export of organic matter from the emergent plant zone.

Submersed macrophytes are limited physiologically by slow rates of diffusion of gases and nutrients in water within the boundary layers surrounding the leaves and by reduced availability of light underwater. Internal recycling of resources, particularly of gases (CO2, O2) of metabolism and of critical nutrients, is important to the abilities of submersed plants to function and grow as well as they do in underwater conditions of chronic light and gas limitations. Despite these adaptive mechanisms, growth and productivity of submersed plants are less than those of emergent and floating macrophytes.

The second most productive component of the wetland–littoral community is the microflora attached to aquatic plants epiphytically and to other surfaces, both living and dead. The surfaces provided by aquatic plants in lakes and rivers can be very large, often exceeding 25 m2 per square meter of bottom sediments. High sustained growth of attached microflora results from their recycling of essential gases (CO2, O2) and dissolved nutrients within the attached communities. Nutrient uptake from the surrounding water is directed primarily to the high net growth of attached microflora and is responsible for the high capacity of wetland–littoral areas to improve the quality of water passing through these communities.

The wetland–littoral complex of higher plant and microbial communities produces the major sources of organic matter and energy of many freshwater ecosystems, including the marginal floodplains of many rivers. Most of the particulate organic matter is decomposed within these interface regions. Organic matter is exported predominantly from these marginal regions as dissolved organic matter to the recipient lake or river (Fig. 3).

Figure 3. Lateral and vertical boundaries of flowing-water ecosystems. The stream ecosystem boundary is defined as the hyporheic/goundwater interface and thereby includes a substantial volume beneath and lateral to the main channel. Vegetation rooted in the hyporheic zone is therefore part of stream ecosystem production. Arrows indicate flow pathways of dissolved organic matter and inorganic solutes derived from plant detritus within the stream ecosystem. [From R. G. Wetzel and A. K. Ward (1992). In Rivers Handbook, I (P. Calow and G. E. Petts, eds.), pp. 354–369. Blackwell Scientific, Oxford, England].

The deep-water pelagic zone of lakes is the least productive along the gradient from land to water (see Fig. 2), regardless of nutrient availability. Growth of phytoplanktonic algae of the pelagic zone is limited by sparse distribution in a dilute environment where efficient nutrient recycling is restricted by the sinking of senescent phytoplankton below the depth of photosynthesis. When nutrient recycling and availability are increased, greater phytoplankton cell densities attenuate underwater light and reduce the volume of water in which photosynthesis occurs. Despite low productivity per unit area, pelagic productivity can be collectively important in large lakes and for higher trophic levels that depend on this source of organic matter.

Higher trophic levels of communities in freshwater ecosystems consist of zooplankton (dominated by four major groups of animals: protozoa/protista, rotifers, and the crustaceans cladocera and copepoda) and benthic invertebrates. In the pelagic zone, small fishes, fry of larger fishes, and predatory zooplankton, which collectively comprise a third trophic level (primary carnivores), consume a portion of these generally herbivorous organisms. A fourth trophic level may consist of medium-sized piscivorous fishes, and the fifth level of large predatory piscivorous fishes. Higher trophic levels are rare in freshwater ecosystems.

The species composition of the higher trophic levels affects the pathways of energy utilization from lower trophic levels. Environmental factors that selectively influence the populations of the communities can alter the pathways and strengths of energy fluxes from subordinate trophic levels. For example, efficiency of consumption of primary production by zooplankton is often appreciably greater in the absence of zooplankton-feeding fishes than in their presence. The population structure of the phytoplankton community responds variably to grazing impacts in concert with their available resources (light, nutrients, and organic constituents). The phytoplankton community may or may not be able to compensate for grazing losses in overall primary production, but generally is able to shift quite quickly to an alternative, less vulnerable species composition.

1. How have ecosystems changed?

1. How have ecosystems changed? – Within terrestrial ecosystems, more than two thirds of the area of 2 of the world's 14 major terrestrial biomes (temperate grasslands Higher levels of threat have been found in the cycads, where 52% are threatened. In general, freshwater habitats tend to have the highest proportion of threatened species.Field Manual for Describing Terrestrial Ecosystems 2nd Edition. 7. Forest Region/District This information can be useful for sorting plot data. Use the following codes: RCO = Coast Forest Region RNI = Northern Interior Forest Region RSI = Southern Interior Forest Region.Terrestrial Ecosystems – Global Land-Based Habitats. The deciduous forest ecosystem is found in temperate regions and experiences temperature and precipitation fluctuations according to four seasons. Desert ecosystems can be hot and dry, semi-arid, coastal or cold.

PDF Describing Terrestrial Ecosystems – The planet's freshwater ecosystems are in crisis: Research found that populations of monitored freshwater species have fallen by 84 "This will require strong policy that recognizes the connections between terrestrial and freshwater systems and that treats those systems as equal in importance."Find the answer to this question here. Super convenient online flashcards for studying and checking your answers!Across a landscape, aquatic-terrestrial interfaces within and between ecosystems are hotspots of organic matter (OM) mineralization. Here, we propose expanding current concepts in aquatic and terrestrial ecosystem sciences to comprehensively evaluate OM turnover at the landscape scale.

PDF  Describing Terrestrial Ecosystems

Ecosystem – Definition, Examples and Types | Biology Dictionary – Freshwater ecosystems: water on land which is continuously cycling and has low salt content (always less than 5 ppt) is known as fresh water. Benthos: The benthic organisms are those found living at the bottom of the water mass. Factors Limiting the Productivity of Aquatic Habitats.Freshwater ecosystems are a subset of Earth's aquatic ecosystems. They include lakes, ponds, rivers, streams, springs, bogs, and wetlands. They can be contrasted with marine ecosystems, which have a larger salt content.An aquatic ecosystem is an ecosystem in a body of water. Communities of organisms that are dependent on each other and on their environment live in aquatic ecosystems. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems.

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Terrestrial vs aquatic ecosystems – .

Ecosystem Essential and Terrestrial Biomes – We are now onto the last topic module
in GEOG/ENVI A111.
This one is called "Ecosystem Essentials and Terrestrial
Biomes". You'll see that actually covers two chapters worth of material.
I'm really going to put the focus on terrestrial biomes, because I think it
really ties into the stuff we just did on the geography of soils, and I think it
really relates to the globally descriptive nature of GEOG A111. But
before we get on with that material, let's just briefly talk about ecosystem
essentials. The first thing to note is that the abiotic components of
ecosystems, which are insolation, water, and gases and minerals, are, to some
extent, things that we've already talked about in this course. The degree to which
things like solar energy and general climate and precipitation and things
like that impact the surface of the earth, it, of
course, impacts living things as well. So we'll talk a little bit about how
different climates help to really structure the different types of
ecosystems that are found around the world. It's also worth noting, following
on the material from the chapter, that there are three basic biotic components
in the trophic structure of an ecosystem: the producers, the consumers,
and the decomposers. In the course of this lecture will really be focused on
the producers and at that, the big producers. The big keystone
aspects of the production cycle in each biome –
generally speaking, large trees or things that have a lot of biomass, like grasses
and things like that. That will really be the focus of what we talk about. If you
want to know more about the trophic structure of ecosystems and relation to
climate and things like that, ENVI A211 is the course
to take which is actually our environmental science class. Now
ecosystems are limited by a lot of things and here's really where we get
the meshing together of ecosystem issues and basic physical geography. Things that
we've already talked about in this course like the amount of sunlight and
the temperature of an area, the amount of water that's available,
these things have a direct impact on the types of ecosystems that are able to
grow or not able to grow. Of course they impact the soil chemistry as well.
So it's really useful to think about climate as we think about ecosystems,
because climate, to some extent, is going to have a strong limiting influence on
the way that ecosystems are able to develop in an area. Just one example
is sunlight. This is a map right here that shows the amount of calories
per square meter that various parts of the world get. You can see, of course,
that sunlight is concentrated in the tropical areas, where we would expect it
to. This in turn has a big impact on the type of plants that can grow there. Now
this map that we're going to look at right here is a map that shows the earth's
terrestrial biomes. It's one that you should keep coming back to as we talk
about each of these various biomes in turn. The first biome, we usually start with
tropical areas, and we're going to do the same thing again here, are tropical rain
forests, which, not surprisingly, are found in tropical rainforest climates. These
are areas that have plentiful insolation, a lot of moisture all year round as well.
So, they have the highest biomass production of any of the biomes.
However, we know they have relatively poor soils because these areas tend to
be heavily laterized – a lot of extra surplus moisture in these areas. So then
the question becomes is: How do the major components of the biome address this? How
do they deal with, from an evolutionary standpoint of course, the
problems and also the possibilities of the conditions in these areas? Here we've got lots of sunlight, lots of moisture, so lots of
ability to grow, but poor soils. Well, what, generally, is the case for most of the
large trees in this part of the world is that they tend to have buttressed trunks,
which means they don't put a lot of their energy into growing deep root
systems, instead they create extensive root systems that spread
over a large area. They form large buttresses that helps to support the
weight of these trees even though they're not able to dig into the soil
very much. In addition to this, you tend to have a lot of stories in your
vegetation. You have big trees that take up a lot of the sunlight, but then you
have a lot of other aspects of the biome that are found in lower levels of the
ecosystem that are able to take advantage of the fact that, even though
sunlight is diffused, there's plentiful moisture and they can still grow. You
also have many many species in this part of the world that are arboreal, which
means they're really dependent on that storied vegetation. Here is a little
diagram that shows some of those buttressed trees and shows the different
canopies that are found in the tropical rainforest, a very dense part of the
world's vegetation. Now once you start moving out of the rainforest and you
start moving into more savanna climates, you still have plentiful insolation, but
you may have seasonal rainfall and that seasonal rainfall can be very stressful
to large plants. So, they often make big adaptations to deal with that. There also is often fire in these
areas as well. So what ends up happening, as you start to move into less and less
plentiful moisture all year round, is you start to move into what's called
tropical forests and scrub areas. These are places where you still have
trees, but you also tend to have a lot of scrubby things growing. Here you have a
lot of plants that have thorns on them. These thorns help to prevent browsing
taking place from the large ungulates that live in the region. You also
have trees that are at least semi- deciduous. Some trees keep their leaves
all year round. depending on how moist it is. others lose them. They often have
really waxy surfaces as well – the leaves do. And there tends to be a lot of gum in
them, too, which helps to prevent and protect them, especially when they are
browsed and when there are problems with them. The soil regimes in these areas are
usually pretty mixed but they're, for the most part, laterized
and so the soils aren't particularly rich for growing. Down below is an image
that shows these areas. If we move to the next image over, we'll see
trees that are often found in that thorny area the dornfeld,
as it's often called. This is actually a particular plant called the
gum arabic plant that is particularly well adapted to these sorts of growing
conditions. Now as you move your way even further out of the tropical rainforest
areas into the true wet dry climates, you eventually get into grassland areas,
where you'll still get trees. You'll still get scrub, but for the most part,
you'll have grasses. Again, mixed soil regimes, but they tend towards laterization,
and here plants start to become more what is called xerophytic, which
means that they are adapted to really pronounced and prolonged dry seasons.
They have small thick and waxy leaves. The grasses that grow in these areas tend to
be well adapted to fire, and they also grow very quickly during that wet season.
But then during the dry season they often die off and just allow their root systems to
survive until they can regrow when the wet season comes again.
This is one of the reasons why these areas can often look extremely dry
during the dry season. They just look almost like desert areas, but then as
soon as the rain comes back to them, everything greens up very quickly. Many
of these areas have large land mammals in them. These big ungulates that are
extremely efficient at eating grasses and converting them into energy are animals
like giraffes and other animals that use the savanna and that we
associate with the savanna. Now moving out of tropical areas into middle
latitude areas, the type of biome that I think we most associated with mid-
latitude areas are the broadleaf and mixed forests. These are areas that have
seasonal insolation. So, unlike, tropical areas, during the wintertime there's not
much sunlight and, as a result, they're the ground is
often quite cold and sometimes frozen, which means there's not a lot of
moisture. So what often happens, especially as we move our way
north in these regions, is that trees become deciduous – that is they lose their
leaves during the summer time. They take advantage of bountiful sunlight and warm
conditions during the summer when they can grow big leaves. They can have a big
canopy, but then when autumn comes the trees all start to change color and they
start to lose their leaves as they prepare for the winter. These are areas
that generally have alfisols and utisols in them so, oftentimes, you have
relatively good soils in these areas. This is not so much the case when we move even
farther north and we start to get into boreal and montane forests. These would
be mountainous areas. These are places where seasonal variation in insolation
is even greater – it is darker and colder, more freezing, often
times because they're snow-covered. Very little moisture is available during the
wintertime at all. Often times the soils are spodosols or worse and, as a
result of that, there's very little nutrition in the soils. The types of
biomes that we find in these areas often involve needle-leafed forests. These are trees
that have, as leaves, very small leafed needles, which minimize the amount of
moisture that they lose, especially during the wintertime. They retain
them all year round so that as soon as there is available moisture and
available sunlight, they can grow very quickly. If we move our way to
the west coast areas in the middle latitudes, we start to get into temperate
rainforests. These are areas that we would find in parts of the Pacific
Northwest, stretching into southeast Alaska. They have seasonal insolation but warmer
temperatures all year round and, generally speaking, abundant
precipitation. We know the areas of the Pacific Northwest are known for not a
lot of rain in terms of the total amount, but many rainy days, lots of mist, small
amounts of rain every single day. Not particularly good soils, but they
take advantage of the fact that there is moisture all year round
to continue that growing. We have a lot of needle leaf type forests that
often are quite vertically structured in ways that are quite similar to the
tropical rainforest areas, but they aren't as completely productive as the
tropical rainforest areas are. Now if we move our way south, along the west coast,
we eventually get to Mediterranean areas. The Mediterranean shrub lands as they're
sometimes known are areas that have a winter growing season for the most
part. They're relatively moist and wet during the winter and they generally
don't experience a lot of cold freezing conditions during the summertime. They're
like deserts with huge soil moisture deficits.
There's calcification in these areas a lot of development of alfisols,
sometimes molisols as well, pretty good areas for agriculture, though
there is a risk of salinization. What plants end up doing in this region,
particularly the big parts of the biome that we are familiar with,
is to form extremely hard leafed, shrubby like plants that are very good at being
drought resistant during the summer. Chaparral is what we often associate
with this sort of an environment. You also get things like tropical (citrus) fruits
that have this real thick leathery surface as a skin protecting the fruit
that's inside, so that fruit is able to make it all the way through the very dry
summer until it eventually can reach the rainy season, where the fruits start to
be dropped on the ground. Most of the plants in these areas are also heavily
fire-adapted as well. I think the classic example of this is the redwood
forests. The seeds of redwood trees actually wait until there's fire until they
germinate, so that they are safe from fire and also the fire triggers them because there's nutrients in the area that they can use for growing
as well. Here's an example of that chaparral, and then I've got just a
couple of other pictures of Mediterranean climates as well.
Grapes here which, are of course, classic Mediterranean fruits, and here are some
guys harvesting the bark of a cork tree, a good example of the really thick outer
layer of a tree, in this case cork. Now also found in mid-latitudes are the
grassland areas. These are what we called steppe climates before. Grassland areas
are areas that are quite arid with big summer moisture deficits but tend to
have really good soils. This is where the molisols are found. So what tends to develop
in these grassland areas are really well-developed root systems – sod – and,
again, big grazing animals, just like we had in the savanna as well. Of course
what sod does in a mid-latitude grassland area is protect an area
against the summer droughts. It still retains its large complicated root
structure that can capture moisture when it does fall during the summer, as
thunderstorms, and then also during the shorter periods when there's a lot of
rain in the spring and the fall. But then these particular grasses are very good
at being frozen as well, because many of these areas are quite cold during the
winter. They have very cold temperatures, high winds, and a lot of
difficulty growing in the winter time, not because of dryness, but because of
the overall temperature. We associate the mid-latitude grasslands with growing
grain and, of course, these are the big breadbasket areas we talked about
before. We're getting close to the end! Now it's time to move on to the really
dry areas – desert areas. There are, of course, both warm and cold deserts but the
plants in warm and cold deserts both use the same basic strategies to grow
and to survive. They're either ephemeral plants, which means they might not grow
and they might not blossom for many years in between rain events, in places
where rain might not come but once every five years or something like that. Other
plants are xerophytic, like we talked about before. They have defensive
strategies that help them to retain water through extremely long periods of
drought. These could be deep roots they could be spreading roots
that help to collect moisture relatively quickly, or very small waxy leaves. Some
plants, like cacti, are able to store water for long periods at a time.Here are just a couple of images showing some of these desert biomes. Then the
final biome that I have here is the Arctic and alpine tundra. There is a very limited
growing seasons in these areas so the biomass production is quite small. Most
of the plants have to grow in permafrost areas that are quite moist in those
active zones. They can't have large root systems. They also can't grow very tall
because there simply isn't enough available sunlight. There's lots of moisture, but
most of that moisture is frozen all year round.
So, we get a lot of low, slow growing herbaceous plants in these particular
climates. So these are the basic biomes around the planet. Again I would just
encourage you to think not only about identifying the biome, but being able to
very clearly identify the strategies that plants use to survive in these
biomes. .

Biology Honors Aquatic Ecosystems Lecture – .