John H. Ryther

Nitrogen, Phosphorus, and Eutrophication in the Coastal Marine Environment

The distribution of inorganic nitrogen and phosphorus and bioassay experiments both show that nitrogen is the critical limiting factor to algal growth and eutrophication in coastal marine waters. About twice the amount of phosphate as can be used by the algae is normally present. This surplus results from the low nitrogen to phosphorus ratio in terrigenous contributions, including human waste, and from the fact that phosphorus regenerates more quickly than ammonia from decomposing organic matter. Removal of phosphate from detergents is therefore not likely to slow the eutrophication of coastal marine waters, and its replacement with nitrogen-containing nitrilotriacetic acid may worsen the situation.

The annual cycle of primary production in the Sargasso Sea off Bermuda

Abstract Gross and net primary production have been measured at bi-weekly intervals for 18 months in the North-western Sargasso Sea 15 miles SE. of Bermuda in 1500 fm of water. Ancillary data include temperature, salinity, phosphate, nitrite, nitrate, disolved oxygen, plant pigments, light penetration and incident radiation. A seasonal cycle of production was observed with high levels in the winter and early spring, low levels in the later spring, summer and early fall. Gross production ranged from 0·13 to 2·00, averaging 0·44 g carbon assimilated/m2/day or 160 g C/m2/year. Net production ranged from 0·05 to 0·83 and averaged 0·20 g C/m2/day or 72 g C/m2/year. Production was closely dependent upon vertical mixing, high levels occuring when the water was isothermal and mixed to or near the depth of the permanent thermocline (400 m), low levels being associated with the presence of a seasonal thermocline in the upper 100 m. Nutrient concentrations were extremely low (maxima of 1·8 μgAN/L and 0·16 μg AP/L as inorganic compounds in the upper 100 m) and showed little seasonal variability. Mixing and enrichment from the permanent thermocline is negligible, production being largely dependent upon the rapid re-cycling of nutrients within the upper 400 m. The environmental factors controlling production which make this region differ from temperate or boreal waters, and which permit high production throughout the winter are: (1) low nutrient concentrations, (2) clear water, (3) relatively high incident radiation in winter, (4) a shallow winter mixed layer and (5) a rapid re-cycling of nutrients, possibly due to higher temperatures.

Organic chelators: Factors affecting primary production in the cromwell current upwelling

Abstract Water upwelling out of the subsurface Cromwell Current at the equator in the eastern Pacific Ocean was found to be high in inorganic nutrients, relatively low in dissolved organic carbon, and supported less phytoplankton growth than water to the north or south. One degree north and south of the equator the dissolved organic carbon concentrations were higher and phytoplankton growth was increased. Enrichment experiments showed that only the addition of a strong chelator or an undefined Zooplankton extract could improve phytoplankton growth in the nutrient-rich, newly upwelled water. We suggest that natural organic chelators, released by organisms as the water ages at the surface, may be partly responsible for the increased phytoplankton growth north and south of the equator.

Annual variations in primary production of the Sargasso sea off Bermuda

Abstract Net and gross primary production, measure at two-week intervals over a period of three years, show annual variations during the winter period (November–April) which are directly related to the degree of mixing of the surface water and may be directly attributed to the severity of the winter as revealed by air temperature and wind strength. Seasonal and year-to-year changes were observed in the ratio net : gross production, as indicated by changes in the relationship between C 14 assimilation and chlorophyll.

Physical models of integrated waste recycling- marine polyculture systems

Abstract A combined tertiary sewage treatment—marine aquaculture system has been developed, tested and evaluated using several different experimental sizes and configurations located both at Woods Hole, Mass. and Fort Pierce, Fla. Domestic wastewater effluent from secondary sewage treatment, mixed with sea water, is used as a source of nutrients for growing unicellular marine algae and the algae, in turn, are fed to oysters, clams, and other bivalve molluscs. Solid wastes from the shellfish are fed upon by polychaete worms, amphipods, and other small invertebrates that serve as food for flounder, lobsters, and other commercially valuable secondary crops. Dissolved wastes excreted by the shellfish and other animals and any nutrients not initially removed by the univellular algae are removed by various species of commercial red seaweeds (Chondrus, Gracilaria, Agardhiella, Hypnea) as a final ‘polishing’ step. The final effluent from the system is virtually free of inorganic nitrogen and is incapable of supporting further growth of marine life or of contributing to eutrophication of the receiving waters. A description of experiments with the above food chains and preliminary results with some alternative approaches are discussed, including a detailed account of the nitrogen mass balance through all of the components of one of the experimental systems.

Nitrogen uptake and storage by the red alga Gracilaria tikvahiae (McLachlan, 1979)☆

Abstract Nitrogen-starved plants of the red alga Gracilaria tikvahiae (McLachlan, 1979) assimilate ammonium-nitrogen very rapidly, doubling their total tissue nitrogen content in 8 h or less. Uptake of nitrate-nitrogen is less rapid. Ammonium uptake is initially the same in the dark as in full sunlight, but the light-exposed seaweeds continue to assimilate the nitrogen longer. N-starved plants soaked in a full nutrient medium including NH 4 + -N for as little as 6 h and returned to unenriched flowing sea water will grow at non-nutrient-limiting rates for as much as 2 weeks before they again become nutrient-deficient and their growth rate declines.

Some aspects of the growth and yield of Gracilaria tikvahiae in culture

Abstract A series of outdoor, continuous-flow seawater cultures (50 l; 0.23 m 2 ) were used to investigate the effects of culture density (kg/m 2 ), nutrient loading (total nitrogen input/day) with both NH 4 + N and NO 3 − N, and turnover rate ( flow rate culture volume ) on the growth and yield of Gracilaria tikvahiae . Although specific growth rates as high as 60% per day were recorded for Gracilaria at low densities (0.4 kg wet wt/m 2 ) in summer conditions, maximum year-round yields were obtained at densities of 2.0–3.0 kg wet wt/m 2 . Above a minimal daily nitrogen loading the yield of Gracilaria was independent of (1) nutrient concentration, (2) nitrogen loading, or (3) whether nitrogen was in the form of NH 4 + N or NO 3 − N, but was (4) highly dependent upon flow rate. The time weighted mean annual production during 1976–1977 was 34.8 g dry wt/m 2 ·day or 127 t/ha·yr based on 12-months continuous operation at near optimal densities and flow rates in the non-nutrient limited culture system.

Nutrients limiting the production of phytoplankton in the Sargasso sea, with special reference to iron☆

Abstract The effects of enriching surface water samples from the Sargasso Sea upon the rate of C 14 assimilation was investigated. The addition of nitrate, phosphate and vitamins, added separately or in combination, had no stimulatory effect, while addition of a trace metal mixture increased C 14 uptake by several-fold. The effective component of the trace metal mix found to be iron, which alone enhanced C 14 uptake for 24 hours but which required the addition of nitrogen and phosphorus to produce a comparable effect of over a three-day period.

Organic carbon and the oxygen minimum in the South Atlantic Ocean

Abstract The relationship between organic matter and the oxygen minimum layer was studied in the southwest Atlantic. Data presented support the premise that the oxygen content of the Antartic Intermediate Water between, 36°S and 10°S, is independent of in situ decomposition of dissolved organic and particulate carbon. Oxygen concentrations were predicted by consideration of changes in salinity alone. It is concluded that the concentration of oxygen and organic carbon is controlled by the mixing of opposing water masses, at depths in excess of several hundred meters.

The ratio of photosynthesis to respiration in marine plankton algae and its effect upon the measurement of productivity

Abstract Simultaneous measurements of photosynthesis in tropical ocean waters using Steemann Nielsens C 14 method and Rileys “light-and-dark bottle” oxygen technique showed wide disagreement, with C 14 values consistently lower by a factor of 10–100. Similar comparisons made in coastal waters, gave good agreement between the two methods. Laboratory experiments suggest that plankton algae respire to a large extent the newly-formed products of their photosynthesis. Thus, in measuring photosynthesis by C 14 uptake, a considerable loss of activity may be incurred through respiration. The magnitude of the error introduced by the respiratory loss of C 14 is proportional to the ratio of respiration to photosynthesis. During exponential growth of Chlamydomonas , respiration is 5–10% of photosynthesis, but in nutrient starved, non-growing, cultures respiration may equal photosynthesis. Under condition of both exponential growth and nutrient starvation, photosynthesis measurements by the C 14 method agree with net photosynthesis (the difference between photosynthesis and as measured by the increase in oxygen in light bottles. C 14 values compare with total photosynthesis, determined from light-and-dark bottle experiments, only when the ratio of respiration to photosynthesis is low (e.g., during active growth). In nutrient-starved cultures, where the ratio approaches unity, C 14 experiments give values which are lower by an order of magnitude.