SCALE AND NONEQUILIBRIUM ECOLOGICAL THEORY

The following notes/quotes are largely based on G.P. Harris. 1986. Phytoplankton Ecology: Structure, Function & Fluctuation. Chapman & Hall, London.

Nonequilibrium

Historical ecological theory based on assumption of equilibrium (steady state) and the concept of plenitude: “that all potential niches were filled at all times.... Thus competitive interactions between organisms were seen as the dominant forces driving evolution (ultimate causes of organic diversity), and [thus presumably] ecological events (proximate causes) also."

However, the environment is patchy (variable) in both space and time. Any given sp. may be limited by environmental factors at some times and by competition at others, and different organisms will perceive the same patchy environment in different ways.

Harris' promotes nonequilibrium theory: "environmental disturbances occur w/ sufficient frequency that competitive exclusion is disrupted." This is equivalent to the Intermediate Disturbance Hypothesis developed for terrestrial ecosystems. Competition generates diversity over evolutionary time but may be unnecessary to maintain diversity on ecological time scales. This is consistent w/ S.J. Gould's punctuated equilibrium model of evolution.

"In a fluctuating environment, the fluctuations themselves become a resource and thus a characterization of the fluctuations is as important as a measurment of the average value of a resource." This necessitates the consideration of temporal and spatial scales.

These observations are one solution to Hutchinson's 'paradox of the plankton', the high diversity of phytoplankton communities [relative to expectations of competitive exclusion based on the equilibrium model].

Scale

"We have tended to see the world in terms of m³ and km, convenient scales for us, but entire universes for organisms such as... phytoplankton."

"What is small and fast for one organism may be large and slow for another...[and] 'noise' at one level may contribute to the predictable behavior of ensemble averages at higher levels.... The differences between FW and marine environments are essentially only those of scale."

Phytoplankton are at the mercy of water movements, span 5 orders of magnitude in cell volume and 3 orders in diameter, and their m ranges from a few doublings×d^-1 to <1 doubling×wk^-1. In these respects phytoplankton more nearly resemble bacteria than higher plants, so we must look at shorter time and smaller spatial scales for understanding.

"[In phytoplankton,] any buffering mechanism to protect against unfavorable conditions must be an intracellular mechanism...[by] respond[ing] in a way that enhances its chances of survival.... The degree of perfection [in response] will depend on the speed of the environmental change and the speed of the biological response." Harris Fig. 5.1

Daily variations in productivity (P-I parameters) may exceed weekly, monthly or even seasonal trends Harris Fig. 8.4

Because biologists do not often measure productivity or environmental variables at such high frequency, most data in the literature are subject to severe aliasing. Harris Fig. 8.6

Given the fast response times of phytoplankton physiology and spp. composition, weekly sampling is the absolute minimum frequency to have any confidence in trends.

Species differences in cell size and consequent capabilities for nutrient uptake and storage result in different competitive outcomes depending on resource patchiness (Harris Fig. 8.7); natural waters generally exhibit simultaneous patchiness on several scales: coexistence?

An important light variable is the ratio of euphotic depth to mixing depth (z_eu/z_m); at z_eu/z_m < 1, plankton become light limited and, although they can acclimate somewhat, often spp. shifts result within days; community response = sum of individual spp. responses Harris Fig. 8.9

Relatively few of the many spp. present may be metabolically active at any given time, and spp. vary greatly in [Chl] per cell, so that the universal practice of normalizing Ps to [Chl] may primarily indicate shifts in relative spp. activities, NOT the physiological changes often ascribed.

Surge uptake and storage mean that most spp. do not need continuous nuts supply to grow rapidly, so high growth rate may coincide w/ physiological symptoms of nutrient depletion.

Total average biomass over a season may be controlled by the amplitude of variability in, e.g., z_eu/z_m, rather than the mean value of the variable. Harris Fig. 9.6

Seasonal Succession

Seasonal cycles of total biomass = smoothed sum of individual spp. abundances, thus former fluctuates less than the latter, usually only one to a few peaks per year (temperate lakes).

Major driving force is summer stratification and spring/fall overturn.

Summer total biomass us. limited by light (z_eu/z_m) in oligotrophic or total P/N in eutrophic waters, while dominant spp. sequentially change and growth rate is always high in dominant (active) spp.

Seasonal patterns are difficult to interpret when (as is often the case) data are too infrequent or too localized vertically (surface samples miss deep Chl maxima) and/or horizontally.

Actual lake data of seasonal variation in total biomass and hypothetical succession w/ two perturbation scenarios are shown in Harris Figs. 12.7, 9.8, respectively.

When large environmental changes occur rapidly, as during spring/fall overturn, community changes are likewise large, but there may be a considerable lag (up to 2 weeks) before total biomass returns to capacity and characteristic spp. assemblages establish. Harris Fig. 10.4

Zooplankton lag phytoplankton response, adding further temporal complexity

Lag times in response: spp. composition not correlated w/ instantaneous conditions.

Nonequilibrium communities and successional sequences are subject to chance recruiting events. "Lakes are ecological islands in a terrestrial sea and movement of spp. between lakes is" [a stochastic process].

However, the large persistent pool of rare spp. acts as a 'rapid-response' seed bank following perturbations. Very little is known about rare spp. since they cannot be easily observed.

Seasonal succession is modified by trophic status; different spp. shifts and number/timing of peaks may occur in eutrophic vs. oligotrophic lakes. (Harris Fig. 12.7)

(Sub)tropical lakes often exhibit little seasonal succession, and where present it tends to be either episodic or cued to the rainy/dry seasons.

Despite all this talk about nonequilibrium, repeatable seasonal patterns do occur, indicating that, at least at longer time scales, natural communities tend toward equilibrium. Harris Fig. 10.5

PHYSICS

"Physical properties are the basis of many of the perturbations which are so important for [phytoplankton ecology]"; Margalef calls this an energy supplement to the community.

The ability to predict biological response to physical forcing in lakes and reservoirs will prove to be a useful management tool.

Spatial and temporal scales of physical processes (turbulent kinetic energy, TKE) are clearly correlated, although basin morphometry sets an upper limit, i.e. small lakes and ponds lack large-scale processes.

"Phytoplankton in mixed layer, experiencing moderately windy conditions, circulate around the mixed layer in a time of 30 min to a few hours", which closely matches many physiological responses.

Many phytoplankton grow during summer stratification in the thermocline, where stability persists for periods of weeks.

Thermocline depth in temperate lakes is proportional the square root of lake diameter (wind fetch), but also a thermocline rarely develops at >80% of the mean total depth of the lake: determines the phytoplankton mixing depth and minimum PFD.

In large lakes (5-10 km at mid-latitudes), large scale, low freq. processes such as Coriolis become important. In the Great Lakes, circulation is a combined function of wind stress, Coriolis, pressure gradient forces, and friction, which may take days to weeks to reach equilibrium following a large perturbation (storm). Such large lakes therefore rarely at equilibrium.

Internal waves (fluctuating depth of pycnocline) and coastal jets (persistent longshore currents) produce complex episodically (5-10 day intervals) reversing currents in the coastal zone of large lakes. Coastal jets concentrate velocity components, and thus pollutants and organisms, parallel and near to shore and minimize the offshore transport of materials except during rapid episodic reversals.

Upwelling of deep, usually cooler & nutrient-rich, water due to wind stress is also possible in larger lakes, and can have profound influence on phytoplankton productivity.

Langmuir cells are one common form of vertical mixing that range from cm to several m deep, depending on wind speed and pycnocline depth (which the cells may partly control). In turbid waters, vertical motion of even a few meters can result in huge fluctuations in PFD on a time scale of minutes, possibly w/ major effect on productivity.

CHEMISTRY

FW chemistry º in situ chemical equilibria + dissolution of rocks (aided by diss. CO₂) + anthropogenic input.

Redfield ratio of elemental composition of phytoplankton = 106C:16N:1P by atoms or 42C:7N:1P by weight; many exceptions exist and depends on growth rate (u) Harris Fig. 8.3

Conservative substances: T_r >> T_mix^H2O (T_r = residence time); incl. most major ions, which thus are intercorrelated and tend to occur in broadly stable ratios; Harris Fig. 4.1, Table 4.2; tend to be fairly evenly distributed in basins, both horiz. & vert.

conservative ions only weakly correlated (over broad ranges) w/ spp. composition; e.g. (1) mono-/divalent cation ratio reasonably predicts spp. comp. for desmids, diatoms, and (2) total dissolved solids (TDS) correlates w/ characteristic spp. clusters of diatoms, chloros & cyanos.

Nonconservative substances: T_r << T_mix^H2O; tend to have uneven distribution in basin, esp. vertical, in part regulated by biota; incl. most of the nutrient elements (C, N, P, S, Si, although C is nearly conservative in oceans).

Macronutrients

Includes elements required in relatively large amounts, usually present at uM+ levels in natural waters

N much more mobile in soils, and T_r^P< T_r^N, so N supply to FW generally exceeds that of P, except in many tropical lakes.

In general, most of the nutrient elements are tied up in biota rather than in dissolved form.

Which is the limiting nutrient is a function of both the pool sizes of the various nutrients as well as their respective turnover rates (t); thus, high cell growth rates may occur even in oligotrophic waters (low nutrient pool size, low biomass) given high nutrient turnover rates.

The ratios of both TN:TP and t_N: t_P positively covary w/ trophic status. Harris Fig. 7.3

The biomass turnover rate, indicated by productivity/biomass (P/B) ratio, is inversely related to biomass Harris Fig. 7.1

1. Carbon

Element needed in greatest quantity by plants; present in complex equilibrium:

diss. inorganic C = DIC = TCO₂ =

[CO₂aq] + [H₂CO₃] + [HCO₃^-] + [CO₃^2-]

<--- decreasing pH shifts equil. --->

HCO₃^- is the most abundant ion in FW (see Fig. 4.1); DIC is the major pH buffer system, keeping most FW between pH 6-9.

CO₂aq typically > air equil. (input from decomposition in soils): TCO₂ controlled by basin geology; excess CO₂ dissolves carbonates: deposited as calcite/marl when CO₂ released to air

Both TCO₂ and the proportion in photosynthetically available forms determines potential productivity of a given sp.; some taxa can use either CO₂ or HCO₃^- but others only CO₂; HCO₃^- use inhibited by high CO₂

DIC "never" limits lake biomass, though spp. composition and Ps may be affected. Beardall Table I; Wetzel Fig. 11-1

In very low alkalinity lakes, pH may increase to >10 during Ps, leaving only a tiny fraction of TCO₂ as CO₂ and thus unfavorable for spp. unable to use HCO₃^-

Changes in the diatom community have been used widely as an indicator of lake acidification, but only a few spp. are strongly impacted by pH & carbonate equilibrium.

Typically, spp. common in eutrophic water tolerant of higher pH than oligotrophic spp.

oligotrophic waters: fairly even vertical dist. of TCO₂; eutrophic: low TCO₂ in euphotic zone (consumed in Ps), high in hypolimnion (sinking plankton decomposition)

2. Nitrogen

NO₃^- dominant in oligotrophic waters; NO₂^- & NH₄⁺ become more abundant in eutrophic waters due to decomposition in anoxic sediments, esp. in summer

In some FW systems, N₂ fixation may at times account for ²/₃ to ³/₄ of total N flux

Due to high mobility in soils and rapid lakewater turnover from rivers & groundwater, DIN high relative to P in FW, thus N normally not limiting in a long term sense; soils usually leach NO₃^-, municipal outfalls mainly NH₄⁺

Denitrification and N₂ release in the anoxic hypolimnion or sediment interface becomes increasingly important in N cycling w/ eutrophication.

Plankton can take up either NO₃^- or NH₄⁺, latter preferred (less energy demand)

3. Phosphorus

PO₄^3- tightly bound to soils thus immobile; terrestrial ecosystems conserve P

Vollenweider (1968): In European Lakes, 45% N comes from underground seepage, whereas 53% of P comes from municipal waste & urban runoff

P is a simpler system than N: no redox conversions involved, though organic P is bioavailable; TP = DIP (PO₄^3-) + DOP (diss. organic P)

%DIP increases with eutrophication, consistent w/ shift from predominantly grazing to detrital metabolism Harris Fig. 7.2

As w/ N, most P tied up in particulate fraction rather than dissolved, but P turnover is very fast (minutes in oligo-, hours to days in eutrophic lakes) & leaches quickly from dead cells

PO₄^3- precipitates w/ Fe³⁺ & Ca²⁺ in oxic conditions, but released from anoxic sediments ® reinforces eutrophication which causes high organic matter sinking, hypolimnion anoxia