Allen W. Knight

Nutrient and light limitation of algae in two Northern California streams

Nutrient‐diffusing subsrates were used to investigate nutrient limitation of attached algal assemblages in a shaded stream and an unshaded stream in northern California. Water from both streams contained low levels of nitrogen (< 14 μg.L−1) and very low N:P ratios (< 2). After 31 days of colonization and growth, attached algal biomass on nitrate‐diffusing substrates was significantly greater than on control substrates in the unshaded stream. Nitrate‐diffusing substrates also supported larger numbers of grazing insects in the unshaded stream. The prostrate diatoms Achnanthes lanceolata Bréb. and Coconeis placentula Ehr. displayed the most consistent positive responses to nitrate enrichment. Nutrient enrichment did not increase the accrual of algal biomass in the shaded stream, but algal biomass was significantly greater at sites located under openings in the tree canopy, implicating light as a limiting factor in this stream. Several Navicula and Nitzschia species, and one unidentified Gomphonema species, were positively associated with higher light levels in the shaded stream. Shade appears to be the primary factor limiting algal growth in small northern California streams, but when its effect is reduced by logging, the inherently low levels of nitrogen in these streams can become limiting.

Experimental Analysis of the Grazing Interaction Between a Mayfly and Stream Algae

The interaction between the grazing mayfly Ameletus validus and periphyton in a small, northern California stream was examined by manipulating the density of the mayfly in flow-through plexiglass channels. Containing natural cobble substrate and located in situ, the channels established an initial gradient of A. validus at 0, 0.5, 1, and 4 times the average density of the mayfly in Barnwell Creek. After 23 d, A. validus significantly depressed periphyton standing crop: ash-free dry mass (AFDM) at the 0, 0.5, 1, and 4 N grazer densities was 5.067 ± 1.389 (se), 1.829 ± 0.173, 1.741 ± 0.325, and 1.009 ± 0.199 g/m2 (ANOVA: P < .01). The mayfly also influenced two structural attributes of the periphyton, increasing the amount of chlorophyll a per unit biomass and decreasing the relative contribution of the loose, upper layer to total periphyton biomass. Principal component analysis of algal relative abundances contrasted the effect of grazing on two groups of diatoms. A group of species found primarily in the loose layer of periphyton (Nitzschia spp., Surirella spiralis, Cymatopleura elliptica, and Navicula cryptocephala) was disproportionately reduced in abundance, while an adnate group (Gomphonema clevei, Achnanthes minutissima, Synedra ulna, Rhoicosphenia curvata, and an undescribed species of Epithemia) increased its relative abundance with increasing grazing pressure. The decline in relative abundance of the loose layer diatoms did not appear to result from selective consumption by A. validus, but may have been mediated by a reduction of inorganic sediment in the periphyton by A. validus. Inorganic sediment was highly correlated with the relative abundances of the loose layer group of diatoms, a group of species that are adapted for locomotion on sediment substrates. A. validus growth in the experimental channels was strongly density dependent. Growth in length over 23 d for the 0.5, 1, and 4 N treatments was 2.24 ± 0.17, 1.80 ± 0.23, and 1.15 ± 0.25 mm (ANOVA: P < .01). The significantly greater growth of A. validus at subnormal densities in the experimental channels suggested that the A. validus population in Barnwell Creek was food-limited.

INTERACTION OF IONIZED AND UN-IONIZED AMMONIA ON SHORT-TERM SURVIVAL AND GROWTH OF PRAWN LARVAE, MACROBRACHIUM ROSENBERGH

1. The toxicity of ammonia to Macrobrachium larvae was tested at pH 6.83, 7.60, and 8.34, and the respective 144 hr LC50 values were 80, 44, and 14 mg ammonia/liter. 2. Toxicity of ammonia was not due solely to the NH3 molecule. In solutions of different pH and equal NH3 concentrations, survival was greatly reduced as NH4+ levels increased. 3. A model is proposed to explain the differential effect of ammonia as pH varies. At higher pH (8.4) toxicity results from copious diffusion of NH3 into larvae. At lower pH (6.8) toxicity is thought to result from competitive inhibition of Na+ transport by NH4+. 4. Retardation of growth was documented in sublethal concentrations of ammonia at 6.8 and 7.6. The average dry weight was about 26% less than that of controls (P < 0.05) after a seven day exposure. 5. Results are discussed relevant to the culture and maintenance of crustaceans, and it is concluded that ammonia will not pose a substantial threat in adequately managed systems.

Effects of androgenic gland ablation on male primary and secondary sexual characteristics in the Malaysian prawn, Macrobrachium rosenbergii (de Man) (Decapoda, Palaemonidae), with first evidence of induced feminization in a nonhermaphroditic decapod

Abstract Male Macrobachium rosenbergii (de Man), categorized into three developmental stages according to possession of male gonopore complexes, appendixes masculina, and mature chelipeds, were subjected to bilateral androgenic gland ablation (andrectomy). Andrectomized males initially lacking appendixes masculina and mature chelipeds do not develop them; those initially possessing appendixes masculina and mature chelipeds do not lose them. Growth rate of the appendixes masculina, however, is reduced. Andrectomized males are unable to redifferentiate amputated or accidentally lost appendixes masculina and mature chelipeds. Instead, the appendages regenerate as immature forms. Andrectomized M. rosenbergii possess atrophied testes and vasa deferentia. Histological sections of the testes revealed a reduction in number of spermatogenic lobules. Spermatogonia and primary spermatocytes were commonly found; secondary spermatocytes and spermatids were found only occasionally. Andrectomy appears to inhibit but not prevent meiosis. Feminization occurred in five andrectomized males. Complete feminization, including initiation of oogenesis and development of oviducts and female gonopores, occurred in males andrectomized in the youngest developmental stage. Males andrectomized in later developmental stages were either partially feminized or not feminized. Reimplantation of androgenic glands reverses the effects of andrectomy. Remasculinization, as evidenced by differentiation of appendixes masculina, was noted after the first postimplantation molt. The testes of remasculinized males were normal in gross anatomy and spermatogenic ability. The data indicate a uniform function for the androgenic glands with respect to male primary and secondary sexual characteristics among the Amphipoda, Isopoda, and Decapoda. Past theories on androgenic gland function are modified in light of the present information.

Chronic toxicity of copper to a partial life cycle of the midge,Chironomus decorus

The development and hatchability ofChironomus decorus eggs were not affected by 0.1 to 5 mg/L of copper in water. The embryos developed normally and hatched at about the same time (after 55 hr of incubation). All larvae survived the duration of the test (72 hr) except those subjected to 5 mg/L of copper in water, which died after only partial emergence from the egg shell. Apparently, eggs were protected by their shell from copper. Growth ofC. decorus larvae was reduced significantly when they were reared in copper-spiked food-substrate (bound copper) from the age 1 to 15 days old (900–4,500 mg/kg of copper). The substrate copper concentration at which larval growth was reduced to 50% (EC50) was 1,602 mg/kg. Substrate copper caused deformities in the epipharyngeal plate of larval mouthparts and copper concentration higher than 1,800 mg/kg delayed adult emergence. The copper concentration in pupal exuviae and adults were positively correlated to copper concentration in the substrate in which they had been reared as larvae. Larval stage appeared to be the most sensitive to copper toxicity, while eggs were the least sensitive. Larval growth was the best indicator in detecting copper pollution, since it could detect copper at relatively low concentrations. The time to adult emergence was not considered a very good indicator, while larval deformities offered a quick tool to evaluate copper pollution.

Masculinization of female Macrobrachium rosenbergii (de Man) (Decapoda, Palaemonidae) by androgenic gland implantation

Abstract Sexually immature and mature female Macrobrachium rosenbergii (de Man) were implanted with vas deferens, testicular, or androgenic gland tissues. Females implanted with vas deferens or testicular tissue developed normally. In contrast, androgenic gland implantation masculinized 81% of female recipients. The first morphological evidence of masculinization was usually differentiation of appendixes masculina. Male gonopore complexes were developed by three sexually immature females and mature male chelipeds were developed by six large masculinized females. Females that were sexually immature at masculinization, i.e., those lacking brood chambers and reproductive setae, did not develop these structures. Females that were sexually mature at masculinization still evidenced brood chambers and reproductive setae at the termination of the experiment. All masculinized females retained their female gonopores. Internally, all but one masculinized female possessed bilaterally formed vasa deferentia. In females possessing male gonopore complexes, the vasa deferentia were completely formed. In the other masculinized females, the vasa deferentia were incompletely formed and existed as autonomous growths along the cephalothorax wall. Two masculinized females initiated spermatogenesis in their ovaries. Spermatogenic lobules were generally better developed in the anterior region of the ovaries. Oogenesis was still present but the oocytes never completed vitellogenesis. In remaining masculinized females, no evidence of spermatogenesis was found. Oogenesis was present but, as in females that had initiated spermatogenesis, the oocytes never completed vitellogenesis. The present results are comparable to past results of androgenic gland implantation experiments and support our hypothesis that the androgenic glands function similarly in the Amphipoda, Isopoda, and Decapoda.

Effects of elevated foodborne selenium on growth and reproduction of the fathead minnow (Pimephales promelas)

Several field studies of selenium-contaminated lakes and reservoirs have indicated the possibility of selenium-induced reproductive failure in important populations of fish. These investigators have hypothesized that bioaccumulation of selenium through the food chain led to fish selenium levels high enough to elicit toxic responses. The present investigation was designed to determine the effects of elevated foodborne selenium on the fathead minnow (Pimephales promelas). Fish were fed a diet spiked with a mixture of inorganic (selenite and selenate) and organic (seleno-L-methionine) selenium and effects on growth and reproduction were determined. Growth was significantly inhibited at the highest selenium treatment levels evaluated (20 and 30 ppm Se). There were no significant treatment effects on any of the reproductive parameters measured. Reasons for the disparity between selenium-induced reproductive impairment observed in other species and apparent lack of impairment in fathead minnows may involve reduced bioaccumulation of selenium by minnows due to differences in gut morphology and physiology.

The effect of phytoplankton and suspended sediment on the growth of Corbicula fluminea (Bivalvia)

Juvenile Corbicula fluminea Müller (1774) were cultured at 15.3 °C in the laboratory on eight combinations of suspended sediment and phytoplankton. Sediment concentrations were 2.6, 25, 50, and 150 mg l−1. Chlorophyll a levels were 15.6 and 62.5 µg l−1. Clam tissue growth was found to be independent of silt concentration but increased at the higher chlorophyll level (p < 0.05). The growth experiment was repeated at 24 °C with chlorophyll a concentrations of 18.9 and 112.6 µg l−1. Growth was again greater at the higher phytoplankton level (p < 0.05). These results demonstrate that Asiatic clam populations are food-limited most of the growing season in the Northern and Western portions of Californias eutrophic Sacramento-San Joaquin Delta where chlorophyll a levels average less than the lower of these values. Comparisons of clam growth in the laboratory and estuary support the food limitation hypothesis as at the higher food concentration laboratory tissue growth was 2.3 and 3.8 times greater during the high and low temperature evaluations than in the estuary.

The effects of temperature and salinity on the metabolic rate of juvenile Macrobrachium rosenbergii (Crustacea: Palaemonidae)

Abstract 1. 1. The respiration of juvenile Macrobrachium rosenbergii were monitored in response to combinations of temperature and salinity encompassing temperatures of 20, 27 and 34°C and salinities of 0, 7, 14, 20 and 28%0. 2. 2. Both oxygen consumption rate and QO2 were influenced by the body weight of the individual prawns. 3. 3. Metabolic rates were determined to be influenced by temperature and salinity and the temperature-salinity interaction. 4. 4. The Q10 values for metabolic rate were influenced by the temperature range considered but there was no obvious response of Q10 to salinity. 5. 5. The metabolic responses to temperature and salinity may determine the distribution and migration of the species in natural habitats.

Acute toxicity of nitrite to larvae of the giant Malaysian prawn, Macrobrachium rosenbergii

Abstract Static bioassays of up to 8 days duration were performed on 10–14-day-old larvae ranging in weight from 80 to 140 μ g/larva. The 24-, 96- and 192-h LC 50 values were 130, 8.6 and 4.5 mg NO 2 -N/l respectively. The highest concentrations of nitrite in which no mortality occurred were 9.7 and 1.4 mg/l in a 24- and 168-h period. There was a significant difference in weight of control larvae compared with those exposed to 1.8 mg/l for 8 days.