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7.2.2 Copepod Population Ecology

Copepods are a subclass of small crustaceans found in marine and freshwater habitats throughout the world, including Arctic and sub-Arctic waters.  Copepods feed on phytoplankton or other zooplankton making them the dominant secondary producer present in the water column.  Their health and abundance can act as a bottom-up control on entire ecosystems, including specific effects on commercially important fisheries such as cod (Beaugrand et al. 2003) as well as populations of marine birds and mammals.  This section presents a broad overview of some aspects of the population ecology of copepods in the Arctic and sub-Arctic including: preferred habitat, life cycle and development, species present, and their respective populations. Each of these descriptions can be used to evaluate the potential impacts to copepod populations after an oil release. Copepod Growth and Development

Copepods are divided into ten orders, but only cyclopoida, poecilostomatoida, and calanoida are common as plankton.  The order calanoida includes some of the most dominant species found in the Arctic in terms of abundance and biomass.  Calanoid copepods all follow the same life stages from egg to adult organism as follows:

  • Egg: Newly laid eggs are small capsules that sink to the bottom and swell until they become spherical.  The number of eggs laid by females of different species is variable.
  • Naupliar stages: The postembryonic development stages are referred to as naupliar stages.  Six such stages (Stages I through VI) exist for calanoid species.  The naupliar stages are characterized by the use of the antennae for swimming and the appearance of a first, unpaired eye.
  • Copepodite stages: A copepodite larva has two pairs of swimming appendages and develops a hind body comprised of the thorax and abdomen.  Five copepodite stages exist (Stages I through V). Molting of the outer exoskeleton marks the transition between each stage.
  • Adult: The adult stage is reached with the development of gonads in males and ovaries in females, with no further molts.

There are common factors such as water temperature and food availability that dictate the growth rate of all copepods.  There are also specific habitat preferences and adaptations that result in varying growth rates for the different species.  Three representative calanoid species, Calanus finmarchicus, Calanus glacialis, and Calanus hyperboreus, are used to illustrate how habitat preferences and seasonal variations in Arctic conditions influence copepod life cycles. Understanding the duration of the copepod life cycle and how their populations shift vertically in the water column in response to food availability can help to inform when a population might be more vulnerable to an oil spill incident.

All species depend on the seasonal production cycle.  The increase of light intensity and melting of sea ice in the Arctic spring produce nutrient rich waters that establish the conditions for a large bloom of phytoplankton.  The carbon fixed by photosynthesis is rapidly stored as lipids by the three Calanus herbivores.  At higher latitudes this cycle of primary production becomes shorter and delayed farther into the summer because of less intense light and differences in ice coverage.  These differences can be significant.  In ice free areas of the SW Barents Sea, the primary production period extends from April through July.  This is followed by continued, but diminished, phytoplankton production into November.  By comparison, the sea ice may not fully melt until July in the in the Kara Sea.  The primary production period extends until mid-September, with reduced production continuing for another month.  Even these time frames are approximations, as there are huge variations in ice cover over time (Falk-Petersen et al. 2009).

Spawning time depends on the species, but occurs before or during the phytoplankton bloom.  Once the eggs have hatched the nauplii rapidly progress through the six stages into copepodites.  For Arctic species the annual bloom ends before copepodites develop into adults.  Copepodites of each species overwinter at depth in a form of hibernation referred to as diapause.  Figure 7-1 presents a summary of the each species response to the spring bloom and corresponding winter diapause.  The transition through the copepodite stages and into adult male and female organisms is marked on the figure.  The following paragraphs detail some species-specific developmental characteristics.

Calanus finmarchicus:  The geographical range of this species is centered in the Norwegian and Labrador Seas as well as in the Barents Sea south of the polar front.  However, it is frequently observed in the Arctic Ocean as an expatriate species (Kosobokova et al. 2011).  C. finmarchicus has a one year life cycle where it spawns at the maximum of the phytoplankton bloom.  Copepodites develop into lipid rich stage V by June-July and then descend to 200 to 1600 meters to undergo diapause.   The stage V copepodites begin to develop into males and females around January.  This process continues through into March when they ascend to the surface waters (Falk-Petersen et al. 2009).

Calanus glacialis:  This is a shelf species that lives in waters that can be ice covered into the summer months.  C glacialis typically has a two year life cycle (Figure 7-1) that begins with spawning during the spring of its third year.  Spawning takes place before or during the Arctic bloom, and likely requires additional energy input from ice algae or fecal pellets of ice dwelling organisms (Sampei et al. 2009). 

Figure 7-1. Upper and lower lines span the depth distribution range. (Source: Falk-Petersen et al. 2009)
Figure 7-1. Upper and lower lines span the depth distribution range. (Source: Falk-Petersen et al. 2009)

Copepodite stage III is reached in the first summer.  Development into stage IV and V and then to adult females require significant lipid reserves and is less likely to occur in the first year.  Instead, the stage III copepodites enter diapause and resume growth with the following bloom.  Once stage V is reached, the copepodites overwinter and develop into females for the following seasons spawning (Falk-Petersen et al. 2009).

Calanus hyperboreus:  These are a large copepod species and one of the Arctic’s key grazers.  Spawning occurs in winter and is fueled by internal lipid reserves.  After the phytoplankton bloom, eggs develop rapidly into stage II and III copepodites.  C. hyperboreus enter diapauses at depths of 800 to 1500 meters as either stage III, IV, or V copepodites.  Over the next two summers, the copepodites grow rapidly through stage IV and into stage V. 

After reaching stage V, the copepodites develop into adults and spawn.  Adult females and stage V copepodites overwinter at different strata.  Females are shallower at 200 to 500 meters where they shed eggs.  Life spans of 1-2 years and 4-6 years have been suggested for C. hyperboreus depending on geographic region and food availability (Falk-Petersen et al. 2009).

In addition to the seasonal migration associated with diapause, copepods can also exhibit a diel migration in an effort to avoid predation.  There is some debate about the extent of this behavior in the Arctic, given both the ice cover and long summer days (Fortier et al. 2001).  However, even when it does occur, the diel migration is not of such a large distance that it significantly changes the vertical position of a copepod population in the water column. Summary of Arctic and Sub-Arctic Copepod Species

Though thousands of copepod species exist throughout the world, relatively few are present in Arctic environments.  Perhaps the best summary of Arctic zooplankton (including copepods) is presented by Kosobokova et al. (2011).  In this study, the authors compiled the results of zooplankton surveys conducted throughout the Arctic Ocean from 1975 through 2007.  A total of 134 locations were sampled at depths ranging from 0 to 3,000 m.  Altogether, 174 different metazoan plankton species were identified from these samples, 91 of which were copepods.  Each species was listed along with their presence or absence in various basins of the Arctic Ocean and their preferred depth range in the water column.  When possible, reproductively active females, eggs, and individuals in the naupliar and copepodite stages were also classified by species.  If some combination of the former were identified, it was considered evidence of a reproducing population in the Arctic.  Species present but not part of reproducing populations were considered expatriates.    Almost 20 percent of Arctic copepods were expatriates.  Six species entered from the Atlantic Ocean, four from the Pacific Ocean, and eight were neritic (Kosobokova et al. 2011).

Arctic copepods were also sorted by their geographic home ranges as a means of determining which species were endemic.  This geographic breakdown is presented in Figure7- 2. Thirty one percent of the identified species were endemic to the Arctic Ocean, including the cryopelagic and newly described Arctic copepods.  The cryopelagic copepods were Jaschnovia tolli, J. brevis, and Eurytemora richingsi.  These three species are most abundant in association with the ice, but occurred with enough frequency at stations where ice was absent for them to be counted as pelagic transients (Kosobokova et al. 2011).

Nineteen percent of the Arctic copepod species could also be found in the North Atlantic compared to only one percent from the North Pacific.  A larger percent of species were present in the North Atlantic and the North Pacific (10 percent) as well as species that were more widely distributed throughout the globe (28 percent). 

The shared distributions of species from the North Atlantic and North Pacific, as well as the presence of expatriate species, are important because it demonstrates the large scale movement of copepods into the Arctic Ocean.  Pacific copepods can enter the Chukchi Sea through an influx of species via the shallow Bering Strait.  In the Atlantic, transport occurs from the Norwegian and Greenland Seas through the Fram Strait, over the shelf of the Barents Sea, and into the Arctic Ocean (Kosobokova et al. 2011).  Although there may be shifts in specific species, copepods in general serve a functional role in the Arctic trophic web (Figure 7-2).

Figure 7-2. Percent of copepods from various geographical ranges out of a total of 91 species. AO = Arctic Ocean, NA = North Atlantic, NP = North Pacific [Source: Kosobokova et al. 2011]
Figure 7-2. Percent of copepods from various geographical ranges out of a total of 91 species. AO = Arctic Ocean, NA = North Atlantic, NP = North Pacific [Source: Kosobokova et al. 2011]